Once again, South Africa finds itself halting use, at least temporarily, of a Covid-19 vaccine it had bet on.

South Africa has faced blow after blow to its pandemic-control efforts: A worrisome variant swept across the country, driving a devastating second wave of coronavirus cases. Then officials had to scramble for an alternative when the vaccine it had bet on, from AstraZeneca, proved ineffective against the variant, which can partially dodge the body’s immune system response.

Now the alternative — Johnson & Johnson’s single-dose vaccine, the only one now in use in South Africa — has run into trouble as well, over concerns of rare blood clots that emerged in a handful of people in the United States who had received the shot. It is unclear whether the vaccine is responsible.

South Africa’s health minister, Dr. Zwelini Mkhize, announced on Tuesday that the country would temporarily halt its vaccine program for medical workers, which has inoculated around 290,000 people so far. Dr. Mkhize said he expected the program — a clinical trial — to resume in a few days, after the authorities have had a chance to look into the blood clot cases in the United States.

“Science must be respected at all times, although this may mean a disruption in our plans,” Dr. Mkhize said on Tuesday.

halted use of the AstraZeneca vaccine after evidence emerged that it did not protect clinical-trial participants from becoming mildly or moderately ill from the variant, known as B.1.351, that is now dominant in the country. South African authorities then pivoted to the Johnson & Johnson vaccine, which is manufactured in the country under license and has a 64 percent efficacy rate in South Africa, according to an analysis by the U.S. Food and Drug Administration.

Health experts say that the decision on Tuesday to pause vaccinating health care workers is the kind of thing that happens often in clinical trials, and that it probably won’t have any major implications for vaccinating the general public.

“At the moment, there is nothing to indicate that this will delay the national rollout program,” said Dr. Richard Lessells, an infectious diseases specialist at the KwaZulu-Natal Research and Innovation Sequencing Platform.

Even so, if evidence emerges to implicate the Johnson & Johnson vaccine in blood clotting problem, and health officials begin to question its safety, it could be a devastating blow for South Africa, the African country hardest hit by the coronavirus, as it races to inoculate its population before an even more dangerous variant appears.

“The U.S. has access to other vaccines to fill a gap, in terms of not using the Johnson & Johnson vaccine,” said Shabir Madhi, a virologist at University of the Witwatersrand who ran the AstraZeneca vaccine trial in South Africa. “That sort of luxury doesn’t exist in other countries, including South Africa.”

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How Worried Should You Be About the J&J Vaccine and Blood Clots?

On Tuesday morning, U.S. federal health regulators recommended a pause in the use of Johnson & Johnson’s Covid-19 vaccine while they investigated six reports of blood clots in women ages 18 to 48. One has died, and a second is hospitalized in critical condition.

As of Monday, 6.8 million people in the United States had received the vaccine without any other serious adverse reactions reported.

Experts have yet to determine to what extent, if any, the vaccine is responsible for the clots. But the investigation follows actions by European regulators who concluded that a vaccine made by AstraZeneca may also be the cause of a similar, extremely rare clotting disorder.

U.S. and European public health experts have emphasized that for most people, the benefits of the Covid vaccines far outweigh the risks.

recommends that people who have received the Johnson & Johnson vaccine within the past three weeks should contact their doctors if they experience severe headaches, abdominal pain, leg pain or shortness of breath. People should not be concerned about mild headaches and flu-like symptoms in the first few days after vaccination. Those are common, harmless side effects brought on by the immune system’s production of a defense against the coronavirus.

During clinical trials and after vaccines go into wide use, experts keep track of any medical problems experienced by people who receive them. If an unusually large cluster of cases turns up, regulators may decide to pause a trial or stop the use of a vaccine to investigate further.

Pauses are common, and typically the investigations reveal that the medical problems were a matter of coincidence. If the investigation reveals that a vaccine does pose a risk, regulators may write new guidance about who should or should not receive it.

regulators have said, roughly one in 1,000 people are affected by a blood clot in a vein every year.

But the clotting disorder of concern in the vaccine recipients is much rarer and different from typical blood clots. In addition to clotting in the brain — called cerebral venous sinus thrombosis, or CVST for short — the patients all had a notably low level of platelets, which left them prone to abnormal bleeding.

said the company was aware of an extremely rare disorder involving people with blood clots in combination with low platelets in a small number of individuals who have received our COVID-19 vaccine. “In addition, we have been reviewing these cases with European health authorities,” the company said in its statement. “We have made the decision to proactively delay the rollout of our vaccine in Europe.”

At the news conference on Tuesday, Dr. Marks of the F.D.A. said the cases were “very, very similar.”

a number of possible symptoms, including swelling in the leg, persistent abdominal pain, severe and persistent headaches or blurred vision, and tiny blood spots under the skin beyond the area where the injection was given.

But that set of symptoms was so vague that almost immediately, British emergency rooms experienced a surge in patients who were worried that they fit the description.

Nonetheless, German researchers say that such symptoms in vaccine recipients must be followed up. Blood tests can detect the antibodies.

Doctors in Germany and Norway have treated patients with blood-thinning drugs to try to stop the growth of the clots, and with intravenous immune globulin, which can help eliminate the misguided antibodies that are causing the problem.

AstraZeneca vaccines. One recipient, a physician in Florida, died from a brain hemorrhage when his platelet levels could not be restored, and others have been hospitalized. U.S. health officials have said that the cases are being investigated, but they have not reported the findings of those reviews and have yet to indicate that there is any link to the vaccines.

Benjamin Mueller contributed reporting.

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Covid-19 Live Updates: U.S. Calls for Pause on Johnson & Johnson Vaccine, Complicating Rollout

Johnson & Johnson’s single-dose coronavirus vaccine after six recipients in the United States developed a rare disorder involving blood clots within about two weeks of vaccination.

All six recipients were women between the ages of 18 and 48. One woman died and a second woman in Nebraska has been hospitalized in critical condition.

Nearly seven million people in the United States have received Johnson & Johnson shots so far, and roughly nine million more doses have been shipped out to the states, according to data from the Centers for Disease Control and Prevention.

“We are recommending a pause in the use of this vaccine out of an abundance of caution,” Dr. Peter Marks, director of the Food and Drug Administration’s Center for Biologics Evaluation and Research, and Dr. Anne Schuchat, principal deputy director of the C.D.C., said in a joint statement. “Right now, these adverse events appear to be extremely rare.”

On a media call later on Tuesday morning, Dr. Marks said that “on an individual basis, a provider and patient can make a determination whether or not to receive the vaccine” manufactured by Johnson & Johnson.

While the move was framed as a recommendation to health practitioners in the states, the federal government is expected to pause administration of the vaccine at all federally run vaccination sites. Federal officials expect that state health officials will take that as a strong signal to do the same. Within two hours of the announcement, Gov. Mike DeWine of Ohio, a Republican, advised all health providers in his state to temporarily stop giving Johnson & Johnson shots. In New York, the health commissioner, Dr. Howard Zucker, said the state would halt the use of the vaccine statewide while federal officials evaluate the safety risks. Appointments for Johnson & Johnson’s shot on Tuesday at state mass sites would be honored with Pfizer doses, Dr. Zucker said.

The authorities in New Jersey, Connecticut, Massachusetts, Maryland, Nebraska, Georgia, Indiana, Texas and Virginia also said that they would follow the call from federal health agencies.

Scientists with the F.D.A. and C.D.C. will jointly examine possible links between the vaccine and the disorder and determine whether the F.D.A. should continue to authorize use of the vaccine for all adults or limit the authorization.

In the media call, federal health officials tried to reassure recipients of Johnson & Johnson’s vaccine while at the same time describing symptoms that they should watch out if they received a shot within the past month.

Dr. Schuchat said that the risk of dangerous blood clots is “very low” for people who received the vaccine more than a month ago.

“For people who recently got the vaccine within the last couple of weeks, they should be aware, to look for any symptoms. If you receive the vaccine and develop severe headaches, abdominal pain, leg pain or shortness of breath, you should contact your health care provider and seek medical treatment,” she said. She emphasized that an emergency meeting of the C.D.C.’s outside advisory committee, which has been scheduled for Wednesday, to discuss how to handle the vaccine in the future is made up of independent experts.

Dr. Janet Woodcock, acting commissioner of the Food and Drug Administration, said she expects the pause in distributing and administrating the vaccine will last for “a matter of days” while officials investigate the cases. Officials also stressed that no serious safety problems have emerged with either of the other two federally authorized vaccines, developed by Pfizer-BioNTech and Moderna.

The move could substantially complicate the nation’s vaccination efforts at a time when many states are confronting a surge in new cases and seeking to address vaccine hesitancy. Regulators in Europe and elsewhere are concerned about a similar issue with another coronavirus vaccine, developed by AstraZeneca and Oxford University researchers. That concern has driven up some resistance to all vaccines, even though the AstraZeneca version has not been authorized for emergency use in the United States.

The vast majority of the nation’s vaccine supply comes from two other manufacturers, Pfizer-BioNTech and Moderna, which together deliver more than 23 million doses a week of their two-shot vaccines. There have been no significant safety concerns about either of those vaccines.

But while shipments of the Johnson & Johnson vaccine have been much more limited, the Biden administration had still been counting on using hundreds of thousands of doses every week. In addition to requiring only a single dose, the vaccine is easier to ship and store than the other two, which must be stored at extremely low temperatures.

Jeffrey D. Zients, the White House Covid-19 response coordinator, said Tuesday the pause “will not have a significant impact” the Biden administration’s plans to deliver enough vaccine to be able to inoculate all 260 million adults in the United States by the end of May. With the Johnson & Johnson setback, federal officials expect there will only be enough to cover fewer than 230 million adults. But a certain percentage of the population is expected to refuse shots, so the supply may cover all the demand.

Mr. Zients said the administration will still “reach every adult who wants to be vaccinated” by the May 31 target.

Federal officials are concerned that doctors may not be trained to look for the rare disorder if recipients of the vaccine develop symptoms of it. The federal health agencies said Tuesday morning that “treatment of this specific type of blood clot is different from the treatment that might typically be administered” for blood clots.

“Usually, an anticoagulant drug called heparin is used to treat blood clots. In this setting, administration of heparin may be dangerous, and alternative treatments need to be given,” the statement said.

In a news release, Johnson & Johnson said: “We are aware that thromboembolic events including those with thrombocytopenia have been reported with Covid-19 vaccines. At present, no clear causal relationship has been established between these rare events and the Janssen Covid-19 vaccine.” Janssen is the name of Johnson & Johnson’s division that developed the vaccine.

In the United States alone, 300,000 to 600,000 people a year develop blood clots, according to C.D.C. data. But the particular blood clotting disorder that the vaccine recipients developed, known as cerebral venous sinus thrombosis, is extremely rare.

All of the women developed the condition within about two weeks of vaccination, and government experts are concerned that an immune system response triggered by the vaccine was the cause. Federal officials said there was broad agreement about the need to pause use of the vaccine while the cases are investigated.

The decision is a fresh blow to Johnson & Johnson. Late last month, the company discovered that workers at a Baltimore plant run by its subcontractor had accidentally contaminated a batch of vaccine, forcing the firm to throw out the equivalent of 13 million to 15 million doses. That plant was supposed to take over supply of the vaccine to the United States from Johnson & Johnson’s Dutch plants, which were certified by federal regulators earlier this year.

The Baltimore plant’s certification by the F.D.A. has now been delayed while inspectors investigate quality control issues, sharply reducing the supply of Johnson & Johnson vaccine. The sudden drop in available doses led to widespread complaints from governors and state health officials who had been expecting much bigger shipments of Johnson & Johnson’s vaccine this week than they got.

A Kent State University student getting his Johnson & Johnson vaccination in Kent, Ohio, last week.
Credit…Phil Long/Associated Press

The authorities in Ohio, New York, New Jersey, Connecticut, Massachusetts, Maryland, Nebraska, Georgia, Indiana, Texas and Virginia said on Tuesday that they would follow the call from federal health agencies to pause the administration of Johnson & Johnson’s vaccine after six women in the United States developed a rare disorder involving blood clots within about two weeks of vaccination.

CVS, the nation’s largest retail pharmacy chain, also said that it would immediately stop its use of Johnson & Johnson vaccinations and was emailing customers whose appointments would be canceled. A spokesman said that CVS would reschedule appointments “as soon as possible.”

Gov. Mike DeWine of Ohio and the state’s chief health official said they were advising all state vaccine providers to temporarily halt use of the single-dose vaccine. New York’s health commissioner, Dr. Howard Zucker, said the state would stop using the Johnson & Johnson vaccine, while the Food and Drug Administration and the Centers for Disease Control and Prevention evaluate the safety risks.

Connecticut health officials said they told vaccine providers to delay planned appointments and give an alternative option if they had the supply.

The C.D.C.’s outside advisory committee has scheduled an emergency meeting for Wednesday.

Jeff Zients, the White House Covid coordinator, said on Tuesday that the pause will not have a significant impact on the country’s vaccination campaign, which has accelerated in recent weeks as a rise in new virus cases threatens a fourth possible surge. Many states have already opened vaccination eligibility to all adults and others plan to by next week.

“Over the last few weeks, we have made available more than 25 million doses of Pfizer and Moderna each week, and in fact this week we will make available 28 million doses of these vaccines. This is more than enough supply to continue the current pace of vaccinations of 3 million shots per day,” Mr. Zients said in a statement.

Even though the reaction to the Johnson & Johnson shot is rare, any questions about the safety of the shots could bolster vaccine hesitancy.

Nearly seven million people in the United States have received Johnson & Johnson shots so far, and roughly nine million more doses have been shipped out to the states, according to data from the Centers for Disease Control and Prevention. The six women who developed blood clots were between the ages of 18 and 48. One woman died and a second woman in Nebraska has been hospitalized in critical condition.

“Right now, these adverse events appear to be extremely rare,” Dr. Peter Marks, director of the Food and Drug Administration’s Center for Biologics Evaluation and Research, and Dr. Anne Schuchat, principal deputy director of the C.D.C., said in a joint statement on Tuesday. “People who have received the J&J vaccine who develop severe headache, abdominal pain, leg pain, or shortness of breath within three weeks after vaccination should contact their health care provider.”

Like many states, New York had already prepared for a significant drop in its supply of the Johnson & Johnson vaccine after federal officials said that supplies would be limited because of a production issue at a Baltimore manufacturing plant. On Friday, Gov. Andrew M. Cuomo said that New York expected to receive 34,900 Johnson & Johnson shots, a decrease of 88 percent from the previous week.

Dr. Zucker, New York’s health commissioner, said that the state would honor appointments made at state-run mass vaccination sites for the Johnson & Johnson vaccine by giving people the Pfizer-BioNTech vaccine instead. That vaccine requires two doses, and it was not immediately clear how the state would handle the additional strain on its supply.

Mayor Bill de Blasio of New York City said that the city would work to reschedule appointments at city-run vaccine sites, giving those people the Pfizer or Moderna vaccines instead.

“Every site has been told this morning to stop giving the J&J shots,” he said at a news conference.

Mr. Cuomo received the Johnson & Johnson vaccine at a public appearance last month in Harlem, which he framed as an effort to boost confidence in that vaccine’s efficacy rate and to address vaccine hesitancy.

Regulators in Europe and elsewhere are concerned about a similar issue with another coronavirus vaccine, developed by AstraZeneca and Oxford University researchers. That vaccine has not been authorized for emergency use in the United States.

Students line up for vaccines at Oakland University on Friday in Rochester, Mich. Coronavirus cases in the state have continued to rise in recent weeks.
Credit…Emily Elconin for The New York Times

The virus is again surging in parts of the United States, but it’s a picture with dividing lines: ominous figures in the Northeast and Upper Midwest, but largely not in the South.

Experts are unsure what explains the split, which doesn’t correspond to vaccination levels. Some point to warmer weather in the Sun Belt, while others suspect that decreased testing is muddying the virus’s true footprint.

The contours of where the virus is resurgent can be drawn around one figure: states that are averaging about 15 new cases a day for every 100,000 people. The 23 states — including Alabama, Mississippi and Arkansas — that have averaged that or fewer over the past week seem to be keeping cases relatively low, according to a New York Times database. Nationally, the country is averaging 21 new cases per 100,000 people.

In the 27 states above that line, though, things have been trending for the worse. Michigan has the highest surge of all, reporting the most drastic increase in cases and hospitalizations in recent weeks. Illinois, Minnesota and others have also reported worrisome increases.

Nationally, reported cases in the United States are growing again after a steep fall from the post-holiday peak in January. In the past two weeks, new confirmed cases have jumped about 11 percent, even though vaccinations picked up considerably, with an average of 3.2 million doses given daily.

Some Southern states, like Alabama and Mississippi, are lagging in vaccinations. Only about 28 percent of people in each state have received at least one shot, according to a New York Times vaccine tracker. Still, case counts continue to drop in both states.

Health experts say cases are rising in the Northeast and Upper Midwest for several reasons, including pandemic fatigue, the reopening of schools and the resumption of youth sports.

Hospitalizations tend to follow the trend line in cases by a few weeks, and have been rising in some states, most notably in Michigan.

Officials are also concerned about the spread of more contagious virus variants, especially B.1.1.7, first identified in Britain. The variant is now the leading source of new coronavirus infections in the United States, the director of the Centers for Disease Control and Prevention said last week.

Just why those factors might affect some states more than others is hard to pinpoint, experts say.

Dr. David Rubin, the director of PolicyLab at the Children’s Hospital of Philadelphia, said warmer weather in Southern states and California was probably playing a role, because it allows people to gather outdoors, with less risk of transmission.

New case reports have fallen by about 11 percent in Georgia over the past two weeks. And in Alabama, new cases are down roughly 29 percent, with a 17 percent decline in hospitalizations.

Some experts say, though, that reduced testing in some states could be obscuring the true picture. Testing in Alabama, for instance, has started to dip, but the share of tests that come back positive has remained high, at 11.1 percent, compared with a nationwide average of 5.1 percent, according to data compiled by Johns Hopkins University.

“People who are symptomatic and go to their provider are going to get a test,” said Dr. Michael Saag, the associate dean for global health at the University of Alabama at Birmingham, but “the desire for people to go get tested just because they want to know what their status is has dropped off dramatically.”

Still, Dr. Saag said, there is probably not a hidden spike in cases in Alabama right now, since hospitalizations in the state remain low.

The first dawn prayers of Ramadan around the Kaaba at the Grand Mosque in Mecca, Saudi Arabia, on Tuesday.
Credit…Amr Nabil/Associated Press

Millions of Muslims on Tuesday began celebrating a second Ramadan in the middle of the pandemic, although in many countries the first day of the holy month offered the promise of a Ramadan with fewer restrictions than last year.

Mosques across the Middle East and other parts of the world were closed for prayer last year, and lockdowns prevented festive gatherings with friends and family. In Jerusalem, for instance, the Old City was largely empty and the Aqsa Mosque compound was closed to the public, as coronavirus cases were surging.

But a large degree of normalcy was back on Tuesday: The Old City’s narrow alleys were crowded, sweet shops were preparing Ramadan desserts, clothing stores were open and the Aqsa compound was welcoming worshipers.

“Last year, I felt depressed and I didn’t know how long the pandemic would last,” said Riyad Deis, a co-owner of a spice and dried fruit shop in the Old City, while selling whole pieces of turmeric and Medjool dates to a customer. “Now, I’m relaxed, I have enough money to provide for my family and people are purchasing goods from my shop — it’s a totally different reality.”

The enthusiasm of some didn’t mean the Ramadan would go as normal. Across several countries in the Middle East, the authorities imposed limitations on customs and festivities, requiring that mosques enforce social distancing and telling worshipers to bring their own prayer rugs and to wear face masks.

In Dubai, Saudi Arabia and Egypt, taraweeh, the optional extra prayers that worshipers can observe at night, were capped at half an hour. No one will also be allowed to spend the night in a mosque, as is common during the last 10 days of Ramadan.

Mosques around the region were also prohibited from serving the fast-breaking meal of iftar or the predawn meal of suhoor. Though Muslims could still gather for those meals with friends and family, the authorities asked them to limit those gatherings this year.

In Jerusalem, Omar Kiswani, the director of Al Aqsa Mosque, said he was overjoyed that the compound was open to worshipers, but still urged caution.

“These are times of great happiness — we hope the blessed Aqsa Mosque will return to its pre-pandemic glory — but these are also times of caution because the virus is still out there,” Mr. Kiswani said.

In Egypt, government officials and prominent television hosts linked to the authorities warned Egyptians of a third wave of infections as Ramadan approached, hinting that another curfew or other lockdown restrictions could be imposed if cases rose.

“If you want the houses of God to remain open,” Nouh Elesawy, an official who oversees mosques at the Egyptian Ministry of Endowments, said earlier this month, “adhere to the precautionary procedures and regulations.”

The Ramadan restrictions may hit the hardest in poor neighborhoods, where residents depend on iftar banquets usually sponsored by wealthy individuals or organizations. For those people, feasting and Ramadan gifts are likely to be rarer, with tourism still at a trickle and many small businesses still suffering from the economic effects of the pandemic.

In Lebanon and Syria, the pandemic has worsened economic crisis that will likely squeeze people’s ability to enjoy the holy month, more than the governments’ limited restrictions aimed at curbing the spread of the coronavirus.

In Syria, where experts say the official infection and death numbers for Covid-19 are far below the reality, the government has few restrictions in place. Worshipers will even be allowed to stand in line inside of mosques to pray together after breaking their fast, the Syrian Ministry of Religious Affairs said.

In Lebanon, which emerged recently from a strict lockdown, shops and restaurants can operate regularly during the day but must offer only delivery service during a nighttime curfew from 9:30 p.m. to 5 a.m.

Global Roundup

Administering a coronavirus vaccine to a frontline worker in New Delhi, last week.
Credit…Rebecca Conway for The New York Times

India said on Tuesday that it would fast-track the approval of vaccines in use in other countries, a move aimed at rapidly increasing the country’s vaccine supply as it battles what is currently the world’s biggest coronavirus outbreak.

The Indian government said that it would grant emergency authorization to any foreign-made vaccine that had been approved for use by regulators in the United States, the European Union, Britain or Japan, or by the World Health Organization. The move had been recommended by a panel of Indian scientists and eliminates a requirement for drug companies to conduct local clinical trials.

“The decision will facilitate quicker access to such foreign vaccines” and encourage imports of materials that would boost India’s vaccine manufacturing capacity, the government said in a statement.

Earlier on Tuesday, India’s top drug regulator granted emergency approval to Sputnik V, the Russian-made vaccine, adding a third vaccine to the country’s arsenal on the same day that health officials recorded 161,736 new coronavirus infections in 24 hours.

It was the seventh straight day that India has added more than 100,000 cases, according to a New York Times database. Only the United States has seen a faster rise in infections during the pandemic.

India has administered about 105 million domestically produced vaccine doses for a population of 1.3 billion, but it is widely believed that the country needs to scale up inoculations rapidly because other measures have failed to control the virus. Many states have reimposed partial lockdowns and weekend curfews. In the country’s financial hub, Mumbai, health officials are racing to erect field hospitals as facilities report shortages of oxygen, ventilators and coronavirus testing kits.

And there is the risk of a superspreading event with the gathering of millions of Hindu pilgrims for the annual Kumbh Mela festival on the banks of the Ganges River, where the authorities say they are powerless to enforce social distancing.

India’s outbreak is reverberating worldwide as its pharmaceutical industry — which was supposed to manufacture and export hundreds of millions of doses of the AstraZeneca vaccine — is keeping most supplies at home. The approval of the Sputnik vaccine, whose first doses are expected to be available for use in weeks, offers hope that India could speed up its inoculation drive.

But it is unclear at this stage whether India will be able to procure significant quantities of other vaccines, including the Pfizer, Moderna and Johnson & Johnson shots in use in the United States. Major Western nations have accumulated much of the global supply of those vaccines and manufacturers are struggling to meet the surging demand.

India will import millions of Sputnik doses from Russia and then begin manufacturing the vaccine domestically, officials said. More than 850 million doses will be made, with some intended for export, Kirill Dmitriev, chief executive of the Russian Direct Investment Fund, a sovereign wealth fund that has financed the vaccine’s development, said in an interview with India’s NDTV channel.

“India is a vaccine-manufacturing hub and our strategic partner for production of Sputnik V,” Mr. Dmitriev said.

India has more than 13.6 million confirmed coronavirus cases, the second most after the United States, and 171,058 deaths, the fourth highest toll.

In other news around the world:

Chancellor Angela Merkel, center, at a cabinet meeting in Berlin on Tuesday. Her government’s proposal on coronavirus restrictions would place half the country over the threshold for lockdown.
Credit…Pool photo by Andreas Gora

BERLIN — Chancellor Angela Merkel’s government moved a step closer on Tuesday to securing the right to force restrictions on areas where the coronavirus is spreading rapidly, overriding state leaders reluctant to take action.

Ms. Merkel and her ministers approved a legislative proposal that would make it easier for the national government to enforce lockdowns and other limits on movement in regions where infection levels pass a set threshold. At current levels, it could lock down more than half of the country.

Under Germany’s decentralized leadership structures, the 16 state leaders have been meeting regularly with the chancellor to agree on nationwide coronavirus response policies. But with different regions experiencing different rates of infection, some state leaders have been reluctant to enforce the agreed limitations, leading to confusion and frustration among many Germans.

“I believe this amendment is as important as it is an urgent decision about how to proceed in the coronavirus pandemic,” the chancellor told reporters after meeting with her ministers.

Parliament still has to debate and approve the proposal, which would take the form of an amendment to the Protection Against Infection Act, and that process is expected to begin this week.

“We are in a situation where an emergency mechanism is necessary,” Ralph Brinkhaus, the leader of the Christian Democratic Union in Parliament, told reporters, before a meeting of his party lawmakers to discuss the amendment.

Under the proposed amendment, the federal government could force stores and cultural institutions to close and enforce limits on the number of people allowed to meet up in any region where infections surpass 100 new cases per 100,000 residents over a period of seven days.

More controversially, the law would also allow Ms. Merkel’s government to order that schools and day care centers close if the number of new infections reaches more than 200 per 100,000 inhabitants. Schools fall under the jurisdiction of the states, and local leaders are reluctant to relinquish that control.

Germany has registered more than three million infections and more than 78,700 deaths from Covid-19 since the virus began moving through the country last spring. It recorded 10,810 new cases of infection on Tuesday, bringing the national rate of infection to more than 140 per 100,000.

The number of patients in intensive care is expected to hit a record this month, as the country struggles to vaccinate enough people to get ahead of the spread of the highly contagious B.1.1.7 variant.

Vaccinations at a mosque in London earlier this month. Britain’s program has reached over 32 million people, more than half the adult population.
Credit…Andrew Testa for The New York Times

Britain has now offered vaccinations to everyone in the country age 50 and older, the government announced late on Monday, and is extending its program to another age group, the latest sign that the national rollout is continuing at pace.

On Tuesday, the authorities opened vaccinations to anyone 45 or older, yet the announcement came with a small hiccup: The website for the country’s National Health Service crashed for a short time after the younger cohort was invited to book appointments online.

The new step in the country’s vaccine rollout comes as the authorities eased several restrictions in England on Monday after months of stringent lockdowns, with pubs and restaurants opened for drinks and dining outside, and nonessential shops once again opening their doors.

Prime Minister Boris Johnson called the moment a “hugely significant milestone” and in a statement thanked those involved with the vaccine rollout. Mr. Johnson said the country was on track to offer all adults a vaccination by the end of July. More than 32 million people across Britain have received their first dose of one of the vaccines, according to government data.

The government said it had also already offered vaccinations to every health or care worker, and to everyone with a high-risk medical condition.

England has also began rolling out the Moderna vaccine, which will be offered as an alternative alongside the Pfizer BioNTech vaccine for those under 30, instead of AstraZeneca’s, which has been the mainstay of Britain’s program so far.

There have been concerns about a possible link between the AstraZeneca vaccine and very rare blood clots, and last week British regulators said an alternative should be provided for younger people. Potential infection still poses much greater risks than any vaccine side effect for all those over 30, they said, and could do so for younger people if cases surged again.

“The Moderna rollout marks another milestone in the vaccination program,” Stephen Powis, the medical director of the National Health Service, said in a statement. “We now have a third jab in our armory.”

The vaccination program, he added, “is our hope at the end of a year like no other” as he encouraged people to book their appointments.

But despite the hopeful vaccine news and the return to public life, the country is still battling new cases of the virus, and a cluster in two London neighborhoods of a worrisome variant first discovered in South Africa has prompted mass testing. Health workers have gone door to door to urge residents to get tested, even if they are not showing symptoms, as dozens of cases have emerged. Similar measures were carried out elsewhere in the city earlier this month.

Studies have shown that the variant contains a mutation that diminishes the vaccines’ effectiveness against it. Dr. Susan Hopkins, the chief medical adviser for the country’s test and trace campaign, said the cluster of cases in parts of South London was “significant.”

“It’s really important people in the local area play their part in stopping any further spread within the local community,” she said in a statement.

Pacific Palace, a dim sum restaurant on a commercial strip in the Sunset Park section of Brooklyn, has seen revenue plunge.
Credit…Victor J. Blue for The New York Times

More than a year after the coronavirus first swept through New York, the streets of Sunset Park in southern Brooklyn reflect the pandemic’s deep and unhealed wounds intertwined with signs of a neighborhood trying to edge back to life.

The sidewalks are filling with shoppers and vendors. More businesses are welcoming customers. But owners still struggle to pay rent and keep their enterprises afloat, while many workers laid off after the city locked down last year remain without jobs.

And while the rate of vaccination in New York has increased significantly, the coronavirus still percolates through this densely packed neighborhood. The ZIP code that includes Sunset Park had the highest rate of positive cases in Brooklyn in early April, nearly double the citywide rate. Some residents have expressed skepticism about the vaccines, spooked by false information circulated over TikTok and other social media.

Adding to the stress is a spate of hate crimes and violence against people of Asian descent in New York and around the country, fed in some cases by racist claims that Asian-Americans are responsible for spreading the virus.

About a third of the residents in Sunset Park have received at least one dose of the vaccine, roughly the same level as the city overall, according to the city health data. But local leaders say they want to push that number much higher.

Kuan Neng, 49, the Buddhist monk who founded Xi Fang Temple on Eighth Avenue, said that people had come to him in recent weeks to express concerns over vaccines.

“Why do I need to do that?” is a common refrain, according to Mr. Kuan, followed by: “I’m healthy now. The hard times are over, more or less.”

“Many people want to delay and see,” Mr. Kuan said, himself included.

The owner of the Cinerama Dome in Hollywood and 15 other movie theaters said it would not reopen after the pandemic.
Credit…Kate Warren for The New York Times

ArcLight Cinemas, a beloved chain of movie theaters based in Los Angeles, including the Cinerama Dome in Hollywood, will permanently close all its locations, Pacific Theaters announced on Monday, after the pandemic decimated the cinema business.

ArcLight’s locations in and around Hollywood have played host to many a movie premiere, in addition to being favorite spots for moviegoers seeking out blockbusters and prestige titles. They are operated by Pacific Theaters, which also manages a handful of theaters under the Pacific name, and are owned by Decurion.

“After shutting our doors more than a year ago, today we must share the difficult and sad news that Pacific will not be reopening its ArcLight Cinemas and Pacific Theaters locations,” the company said in a statement.

“This was not the outcome anyone wanted,” it added, “but despite a huge effort that exhausted all potential options, the company does not have a viable way forward.”

Between the Pacific and ArcLight brands, the company owned 16 theaters and more than 300 screens.

The movie theater business has been hit particularly hard by the pandemic. But in recent weeks, the majority of the country’s largest theater chains, including AMC and Regal Cinemas, have reopened in anticipation of the slate of Hollywood films that have been put back on the calendar, many after repeated delays because of pandemic restrictions. A touch of optimism is even in the air as a result of the Warner Bros. movie “Godzilla vs. Kong,” which has generated some $70 million in box office receipts since opening over Easter weekend.

Still, the industry’s trade organization, the National Association of Theater Owners, has long warned that the punishing closures were most likely to affect smaller regional players like ArcLight and Pacific. In March, the Alamo Drafthouse Cinema chain, which operates about 40 locations across the country, announced that it had filed for Chapter 11 bankruptcy protection but would keep most of its locations operational while it restructured.

That does not seem to be the case for Pacific Theaters, which, according to two people with knowledge of the matter, fired its entire staff on Monday.

The reaction to ArcLight’s closing around Hollywood has been emotional, including an outpouring on Twitter.

Firefighters at the site of COVID-19 hospital Matei Bals, after a fire broke out in one of its buildings in Bucharest, Romania, in January.
Credit…Robert Ghement/EPA, via Shutterstock

Three people infected with the coronavirus died at a hospital in Bucharest on Monday evening after the oxygen supply stopped functioning, according to the authorities, the latest incident involving oxygen failure, which in many countries has driven up the virus death toll.

It was also another fatal setback for Romania’s ageing and overwhelmed health care system, which has suffered two fires in Covid-19 wards in recent months, killing at least 15 people.

Ventilators shut down at a mobile intensive care unit set up at the Victor Babes hospital in Bucharest after oxygen pressure reached too high a level, the country’s health authorities said in a statement, depriving patients of a vital supply. In addition to the three patients who died, five others were evacuated and moved to other facilities in the city.

Romania has recorded its highest rate of Covid-19 patients in intensive care units since the pandemic began, and on Sunday Prime Minister Florin Citu said that there were just six intensive care beds available across Romania, out of nearly 1,600.

Intensive care units in Hungary and Poland have also been at risk of being overwhelmed, as much of Eastern Europe has struggled to cope with a third wave of infections across the continent. Some Hungarian hospitals have sought medical students and volunteers to assist in Covid-19 wards, giving training to those without previous medical experience.

The mobile unit struck by the oxygen problem on Monday had only been in operation since Saturday, and it has epitomized long-running concerns over the country’s fragile health care system. In January, five patients died and a further 102 were evacuated from a different hospital in Bucharest after a fire broke out. In November, 10 patients hospitalized with the coronavirus died after a fire broke out in a hospital in the northeastern city of Piatra Neamt.

Romania’s spending on health care is among the lowest in the European Union, with just over five percent of gross domestic product allocated toward it, compared with 10 percent on average among other countries of the bloc.

More than 25,000 people who tested positive for the virus have died in Romania, and the authorities have closed schools and kindergartens throughout April as part of an extended Easter holiday.

The authorities have so far administered more than 3.5 million vaccine doses, in a population of about 19 million.

Alisa Stephens, a biostatistician at the University of Pennsylvania in Philadelphia, had to manage work and taking care of her children after the city went into lockdown last year.
Credit…Hannah Yoon for The New York Times

Studies have found that women in academia have published fewer papers, led fewer clinical trials and received less recognition for their expertise during the pandemic.

Add to that the emotional upheaval of the pandemic, the protests over structural racism, worry about children’s mental health and education, and the lack of time to think or work, and an already unsustainable situation becomes unbearable.

Michelle Cardel, an obesity researcher at the University of Florida, worries that this confluence of factors could push some women to leave the sciences.

“My big fear is that we are going to have a secondary epidemic of loss, particularly of early career women in STEM,” she said.

Female scientists were struggling even before the pandemic. It was not unusual for them to hear that women were not as smart as men, or that a woman who was successful must have received a handout along the way, said Daniela Witten, a biostatistician at the University of Washington in Seattle.

Women in academia often have little recourse when confronted with discrimination. Their institutions sometimes lack the human resources structures common in the business world.

Compounding the frustration are outdated notions about how to help women in science. But social media has allowed women to share some of those concerns and find allies to organize and call out injustice when they see it, said Jessica Hamerman, an immunologist at the Benaroya Research Institute in Seattle.

In November, for example, a study on female scientists was published in the influential journal Nature Communications suggesting that having female mentors would hinder the career of young scientists and recommending that young women seek out male help.

The response was intense and unforgiving: Nearly 7,600 scientists signed a petition calling on the journal to retract the paper — which it did on Dec. 21.

The study arrived at a time when many female scientists were already worried about the pandemic’s effect on their careers, and already on edge and angry with a system that offered them little support.

Alisa Stephens found working from home to be a series of wearying challenges. Dr. Stephens is a biostatistician at the University of Pennsylvania, and carving out the time and mental space for that work with two young children at home was impossible.

Things eased once the family could safely bring in a nanny, but there was still little time for the deep thought Dr. Stephens had relied on each morning for her work.

Over time, she has adjusted her expectations of herself. “Maybe I’m at 80 percent as opposed to 100 percent,” she said, “but I can get things done at 80 percent to some extent.”

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Two worrisome variants make up over 70 percent of cases analyzed in N.Y.C., city officials say.

New York City health officials said on Monday that infections with the coronavirus variant that first emerged in Britain, B.1.1.7, have been increasing in every borough, but slightly more in southern Brooklyn, eastern Queens, and Staten Island. Genetic analysis shows that B.1.1.7 now accounts for about 30 percent of cases sequenced citywide.

The data, which was included in new maps and a report released by the city, represents the first time officials have offered a ZIP-code level look at how worrisome variants have been spreading in New York, overtaking original versions of virus and clustering in some parts of the city more than others.

The report and maps, which were published Monday afternoon on the city’s health department website, also show that a variant first emerged in New York City, B.1.526, has been increasing at even a faster clip, and now represents some 45 percent of cases genetically sequenced in the city. The maps released Monday show that while B.1.526 is found in all five boroughs, it is slightly more common in the Bronx and parts of Queens.

Overall, more than 70 percent of genetically sequenced coronavirus cases now circulating in the city represent worrisome variants. The data, which spans January 1 to March 27, represents less than 5 percent of all positive test results in the city, as sequencing capabilities remain limited. As a result, it only provides a glimpse of the full picture of how the variants are impacting each community.

has remained at a high plateau of coronavirus cases since February, with some 3,000 to 4,000 new cases reported per day, according to city data. The spread of these variants is likely a key reason that cases have not fallen more even as vaccinations rise, the city’s health department said in the report.

Hospitalizations have been falling, but very gradually, as the most vulnerable get vaccinated. Deaths have also been declining, but at a slower than desired pace, and have been averaging about 50 per day.

The United States has seen an exponential rise of B.1.1.7, which is now the most dominant variant across the country, according to the Centers for Disease Control and Prevention. That variant is about 60 percent more contagious and 67 percent more deadly than the original form of the coronavirus, according to the most recent estimates.

It has slammed Europe and helped fuel the worst-in-the-nation outbreak in Michigan. Until recently, the variant’s rise in the United States was somewhat camouflaged by falling infection rates overall, leading some political leaders to relax restrictions on indoor dining, social distancing and other measures. The C.D.C.’s efforts to track down the variants have greatly improved in recent weeks and will continue to grow, though Britain, which has a more centralized health care system, began a highly promoted sequencing program last year that allowed it to track the spread of the B.1.1.7 variant.

Vaccines do appear to be effective against the variant.

Less is known about the B.1.526 variant, which was first documented by researchers in the Upper Manhattan area of New York City last November and has since spread widely through the city and beyond. City officials have said that the variant may be more transmissible, and is outpacing even B.1.1.7 in some neighborhoods.

appeared in many states.

The city’s report did not mention the B.1.351 variant, first found in South Africa, which can partially dodge the body’s immune system response. The city had previously reported sequencing 6 total cases of B.1.351.

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Covid-19 Live Updates: Regeneron’s Antibody Drug Can Help Prevent Infections, Study Says

clinical trial results announced on Monday. The drug, if authorized, could offer another line of defense against the disease for people who are not protected by vaccination.

The findings are the latest evidence that such lab-made drugs not only prevent the worst outcomes of the disease when given early enough, but also help prevent people from getting sick in the first place.

Using the cumbersome drugs preventively on a large scale won’t be necessary: Vaccines are sufficient for the vast majority of people and are increasingly available.

Still, antibody drugs like Regeneron’s could give doctors a new way to protect high-risk people who haven’t been inoculated or who may not respond well to vaccination, such as those taking drugs that weaken their immune system. That could be an important tool as rising coronavirus cases and dangerous virus variants threaten to outpace vaccinations.

Regeneron said in a news release that it would ask the Food and Drug Administration to expand the drug’s emergency authorization — currently for high-risk people who already have Covid but are not hospitalized — to allow it to be given for preventive purposes in “appropriate populations.”

There’s “a very substantial number of people” in the United States and globally who could be a good fit to receive these drugs for preventive purposes, said Dr. Myron Cohen, a University of North Carolina researcher who leads monoclonal antibody efforts for the Covid Prevention Network, a National Institutes of Health-sponsored initiative that helped to oversee the trial.

“Not everyone’s going to take a vaccine, no matter what we do, and not everyone’s going to respond to a vaccine,” Dr. Cohen said.

Regeneron’s new data come from a clinical trial that enrolled more than 1,500 people who lived in the same household as someone who had tested positive for the virus within four days. Those who got an injection of Regeneron’s drug were 81 percent less likely to get sick with Covid compared to volunteers who got a placebo.

Dr. Rajesh Gandhi, an infectious diseases physician at Massachusetts General Hospital who was not involved in the study, said the data were “promising” for people who have not yet been vaccinated. But he said that the study did not enroll the type of patients that would be needed to assess whether the drug should be used preventively for immunocompromised patients. “I would say we don’t yet know that,” Dr. Gandhi said.

Regeneron’s cocktail, a combination of two drugs designed to mimic the antibodies generated naturally when the immune system fends off the virus, got a publicity boost last fall when it was given to President Donald J. Trump after he got sick with Covid.

The treatment received emergency authorization in November. Doctors are using it, as well as another antibody cocktail from Eli Lilly, for high-risk Covid patients.

But use of the antibody drugs has been slowed not by a shortage of doses, but by other challenges, though access has improved in recent months. Many patients don’t know to ask for the drugs or where to find them.

Many hospitals and clinics have not made the treatments a priority because they have been time-consuming and difficult to administer, in large part because they must be given via intravenous infusion. Regeneron plans to ask the F.D.A. to allow its drug to be given via an injection, as it was administered in the results of the study announced on Monday, which would allow it to be given more quickly and easily.

Decorating the exterior of an Italian restaurant in London on Sunday. Pubs and restaurants were permitted to reopen outdoor spaces on Monday.
Credit…Andrew Testa for The New York Times

Britain reopened large parts of its economy on Monday, allowing people in England back in shops, hair salons and outdoor areas of pubs and restaurants, a long-awaited milestone after three months of lockdown, and a day after the country recorded its lowest daily coronavirus death toll since September.

Under the second stage of the government’s gradual reopening, libraries, community centers and some outdoor attractions like zoos will also return, though outdoor gatherings remain limited to six people or two households.

For many in England, the return was a hopeful — if not definitive — sign that the worst of the pandemic was behind them, after a new variant of the virus detected last year in the country’s southeast spun out of control around Christmas, overwhelming hospitals and causing tens of thousands of deaths.

At its winter peak, Britain reported as many as 60,000 daily cases a day and 1,820 daily deaths, according to a New York Times database. But after months of restrictions and an aggressive vaccination program that has offered a dose to about half of Britain’s population, those figures declined to 1,730 daily cases and seven deaths reported on Sunday.

Prime Minister Boris Johnson has so far gone ahead with the gradual easing of measures that he had announced, reopening schools on March 8, reducing restrictions on outdoor gatherings on March 29, and allowing large parts of the economy to reopen on Monday.

Mr. Johnson said on Monday that the reopening was “a major step forward in our road map to freedom.” Still, he urged caution.

“I urge everyone to continue to behave responsibly and remember ‘hands, face, space and fresh air’ to suppress Covid,” he said.

Scotland, Wales and Northern Ireland, where devolved governments are responsible for coronavirus restrictions, have laid out similar plans to reopen their economies.

The apparent success represents a turnaround for Mr. Johnson’s government, which struggled to stem cases earlier in the pandemic and at one point reported the greatest rate of excess deaths in Europe.

But now E.U. countries — hampered by a vaccine rollout slower than Britain’s and a scare over a possible links between the AstraZeneca vaccine and blood clots — are facing a third wave of coronavirus infections. France, Italy and other countries have recently imposed new lockdown measures.

In England, business owners reopened on Monday with hope — and some anxiety that the numbers of infections could go up again. Still, “we’re looking confident we won’t be seeing anything like that again,” said Nicholas Hair, the owner of The Kentish Belle, a London pub that opened its doors to patrons one minute after midnight.

Global Roundup

A train station in Mumbai, on Monday.
Credit…Niharika Kulkarni/Reuters

Even as India hit a record for daily coronavirus infections, and its total caseload rose to second in the world behind the United States, the images that dominated Indian news media on Monday were of a crowded religious festival along the banks of the Ganges River.

The dissonance was a clear manifestation of the confusing messages sent by the authorities just as India’s coronavirus epidemic is spiraling, with a daily high of 168,000 cases and 900 deaths reported on Monday.

Yet millions of devotees have thronged the holy city of Haridwar for the monthlong Kumbh Mela, or pitcher festival, when Hindu pilgrims seek absolution by bathing in the Ganges. Officials have said that about one million people will participate every day, and as many as five million during the most auspicious days, all crowded into a narrow stretch along the river and searching for the holiest spot to take a dip.

Already, fears are running high that one of the most sacred pilgrimages in Hinduism could turn into a superspreading event.

Dr. S. K. Jha, a local health officer, said that an average of about 250 new cases had been registered each day recently. Experts have warned that many more infections are going unrecorded, and that devotees could unwittingly carry the virus with them as they return to their homes across the country.

India is in the grip of the world’s fastest growing outbreak, with more and more jurisdictions going back into varying stages of lockdown. Infections are spreading particularly fast in Mumbai, the country’s financial hub, and the surrounding state of Maharashtra, where the government has announced a partial weekday lockdown and near-total closure over the weekends.

The situation is also worsening in the capital, New Delhi, which reported more than 10,000 new cases on Sunday, surpassing the previous daily high of nearly 8,500. The state government has imposed a curfew and ordered restaurants and public transport systems to run at half capacity. Arvind Kejriwal, Delhi’s top official, has said more restrictions may follow.

Hospitals in several states are reporting shortages of oxygen, ventilators and coronavirus testing kits, and some are also running low on remdesivir, a drug used in serious Covid-19 cases. India has halted the export of remdesivir until the situation improves.

India is also trying to ramp up its vaccination drive, with about three million people being inoculated daily and 104 million doses administered so far. But with many vaccination centers nationwide expressing concern over possible shortages, India’s large pharmaceutical industry has sharply reduced its exports of the AstraZeneca vaccine in order to keep more doses at home, creating serious challenges for other countries that had been relying on those shipments.

On Monday, Indian experts recommended the use of Russia’s Sputnik-V coronavirus vaccine, which would become the third available in the country if approved by the authorities.

After months of lower-than-expected infections and deaths from the virus, critics say Indian officials have sent dissonant messages about the seriousness of the crisis. Police officers are enforcing curfew and mask rules, sometimes resorting to beatings captured on videos shared across social media. But senior political leaders, including the prime minister, Narendra Modi, have been holding large rallies for local elections.

Mr. Modi’s Hindu nationalist government has also allowed the religious festival to proceed — in contrast to what happened last spring, at the start of the pandemic, when India’s health ministry blamed an Islamic seminary for fanning a far smaller outbreak. Critics say rhetoric from members of Mr. Modi’s party contributed to a spate of attacks against Muslims, a minority of about 200 million people in a Hindu-dominated country of 1.3 billion.

In other news around the world:

Anna Schaverien, Constant Méheut and Niki Kitsantonis contributed reporting.

A vaccination center at the Royal Exhibition Building in Melbourne, Australia, last month.
Credit…James Ross/EPA, via Shutterstock

Australia has given up on the goal of vaccinating its entire population against Covid-19 by the end of the year, following updated advice from health officials that younger people should not receive the AstraZeneca vaccine, as well as delays in the delivery of doses.

The Australian government said last week that it had accepted a recommendation by a panel of health experts that people under 50 receive the Pfizer-BioNTech vaccine instead of the one developed by AstraZeneca, which had been the centerpiece of Australia’s vaccination program. The change in guidance came after European regulators found links between the AstraZeneca vaccine and rare blood clots, prompting several countries to restrict use of the shot.

Prime Minister Scott Morrison said Friday that the government had ordered another 20 million doses of the Pfizer vaccine, doubling what it had already purchased. But they are not expected to be available until the fourth quarter of this year, dealing a blow to the government’s previously stated goal of inoculating all of its 25 million people by then.

Mr. Morrison appeared to acknowledge the change in timeline in a Facebook post on Sunday.

“The government has also not set, nor has any plans to set any new targets for completing first doses,” Mr. Morrison said. “While we would like to see these doses completed before the end of the year, it is not possible to set such targets given the many uncertainties involved.”

Public health experts have criticized Mr. Morrison’s government for relying too heavily on the AstraZeneca vaccine, a relatively cheap and easy-to-use shot but one whose troubles have jeopardized inoculation efforts in multiple countries. They said the setback to Australia’s vaccination program risked undermining the country’s success in containing the spread of the coronavirus since recording its first case in January 2020.

“We’re in a position a year later where that hard-won success is jeopardized by a completely incompetent approach to a vaccine rollout,” said Bill Bowtell, a public health policy expert and adjunct professor at the University of New South Wales in Sydney.

Australia has made four separate agreements for the supply of Covid-19 vaccines that would give it a total of 170 million doses, enough to inoculate its population more than three times over. Plans to manufacture almost all of its 54 million AstraZeneca doses domestically were approved last month.

But the Australian government has been under fire for weeks over the sluggish pace of its vaccination rollout, which began in late February. By the end of March, when the government had aimed to vaccinate four million people, only about 600,000 had actually been inoculated. As of Sunday, Australia had administered fewer than 1.2 million doses.

Australian officials have attributed the slow rollout to delays in the delivery of millions of vaccine doses manufactured in the European Union, which has curbed exports amid its own supply shortages. The export restrictions mainly affect the AstraZeneca vaccine.

After enduring strict lockdowns for much of the past year, Australians are now enjoying relatively normal life in a country that has all but stamped out the virus. But public health experts warn that until more of the population is vaccinated, those freedoms are precarious.

“Having eliminated Covid, they thought a mass vaccination campaign would lock that in,” Mr. Bowtell said of the Australian public. “Now they are being deeply disillusioned.”

Covid-19 vaccinations at a monastery in Bangkok this month.
Credit…Adam Dean for The New York Times

Thailand is facing its worst coronavirus outbreak just as millions of people head to their home provinces during the country’s biggest travel holiday.

The latest wave of infections, which has sent at least eight cabinet members into isolation, is centered in a Bangkok nightlife district said to be popular with government officials and wealthy partygoers. The country, which until now has largely kept the virus under control, set a record Monday for new daily cases with 985.

One top health official warned that Thailand could soon face as many as 28,000 new cases a day in the worst-case scenario. The government announced it would set up field hospitals as Covid-19 wards at existing facilities begin to fill up.

Officials ordered the closure of hundreds of bars and nightclubs, but critics say the government has been inconsistent in its efforts to bring the outbreak under control. The prime minister, Prayuth Chan-ocha, stopped short of banning travel between provinces for the Songkran holiday, which begins on Tuesday and marks the beginning of the Thai New Year.

“Whatever will be, will be,” he said last week in explaining his decision. “The reason is it’s a matter that involves a huge number of people. The government will have to try to cope with that later.”

Dozens of provinces have imposed their own restrictions on travelers coming from Bangkok and other affected areas, prompting many Thais to cancel their trips. But many others set off over the weekend.

During earlier outbreaks, the government often acted quickly to require face masks, ban foreign tourists, impose quarantine restrictions and lock down hard-hit areas. It has reported fewer than 34,000 cases — mostly from a January surge traced to a seafood market near Bangkok — and just 97 deaths.

But it has been lax in testing and slow to vaccinate. So far, it has procured about 2.2 million doses and given at least one to about 500,000 people. Thailand’s population is 70 million.

Vaccine production is not expected to begin in earnest until June, when a manufacturer in Thailand is scheduled to begin producing 10 million doses a month of the AstraZeneca vaccine.

Health officials were alarmed by the recent discovery of dozens of cases of the highly infectious coronavirus variant first identified in Britain. The finding highlighted the inadequacy of Thailand’s virus testing and suggested that its quarantine procedures have not been as effective as officials believed.

Tourism operators have been especially angered by the government’s lackadaisical approach to obtaining vaccine supplies. The tourism industry, which normally accounts for about 20 percent of the nation’s economy, is highly dependent on foreign visitors and has been calling for widespread vaccinations to speed its recovery.

The outbreak in Bangkok has also prompted questions about the activities of some top officials and their aides.

The transportation minister, Saksayam Chidchob, who was hospitalized with Covid-19, was criticized for not being forthcoming about his whereabouts during times when he may have been exposed to the virus. He denied visiting the gentlemen’s club at the center of the outbreak and said he believed he had contracted the virus from an aide.

Eyan Gallegos, 11, a middle schooler in Washington, completing his homework in his room.
Credit…Gabriella Demczuk for The New York Times

Parents with school-age children have struggled to combine their usual work and family responsibilities this past year with at least some degree of home-schooling.

But mothers and fathers of middle-schoolers — the parenting cohort long known to researchers as the most angst-ridden and unhappy — are connecting now in a specific sort of common misery: the pressing fear that their children, at a vital point in their academic and social lives, have tripped over some key developmental milestones and may never quite find their footing.

Experts say some of their worries are justified — up to a point. The pandemic has taken a major toll on many adolescents’ emotional well-being.

Yet as the nation begins to pivot from trauma to recovery, many mental-health experts and educators are trying to spread the message that parents, too, need a reset. If adults want to guide their children toward resilience, these experts say, then they need to get their own minds out of crisis mode.

Early adolescence is considered a critical period, a time of brain changes so rapid and far-reaching that they rival the plasticity and growth that take place in the newborn to 3-year-old phase.

These changes make children more capable of higher-level thinking and reasoning. They also make them crave social contact, attention and approval.

Remote learning and social distancing are in many ways the opposite of what children in this age group want and need.

It’s been hardest on middle schoolers,” said Phyllis Fagell, a therapist and school counselor who wrote the 2019 book “Middle School Matters.” “It is their job to pull away from parents, to use these years to really focus on figuring out where they are in the pecking order. And all of that hard work that has to happen in these years was just put on hold.”

Yet Ms. Fagell and many other experts in adolescent development were adamant that parents should not panic — and that the spread of the “lost year” narrative needed to stop.

Getting a full picture of what’s going on with middle schoolers, they agreed, requires holding two seemingly contradictory ideas simultaneously in mind: The past year has been terrible. And most middle schoolers will be fine.

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Kati Kariko Helped Shield the World From the Coronavirus

She grew up in Hungary, daughter of a butcher. She decided she wanted to be a scientist, although she had never met one. She moved to the United States in her 20s, but for decades never found a permanent position, instead clinging to the fringes of academia.

Now Katalin Kariko, 66, known to colleagues as Kati, has emerged as one of the heroes of Covid-19 vaccine development. Her work, with her close collaborator, Dr. Drew Weissman of the University of Pennsylvania, laid the foundation for the stunningly successful vaccines made by Pfizer-BioNTech and Moderna.

For her entire career, Dr. Kariko has focused on messenger RNA, or mRNA — the genetic script that carries DNA instructions to each cell’s protein-making machinery. She was convinced mRNA could be used to instruct cells to make their own medicines, including vaccines.

But for many years her career at the University of Pennsylvania was fragile. She migrated from lab to lab, relying on one senior scientist after another to take her in. She never made more than $60,000 a year.

was published, in Immunity, it got little attention.

Dr. Weissman and Dr. Kariko then showed they could induce an animal — a monkey — to make a protein they had selected. In this case, they injected monkeys with mRNA for erythropoietin, a protein that stimulates the body to make red blood cells. The animals’ red blood cell counts soared.

25 years of work by multiple scientists, including Pieter Cullis of the University of British Columbia.

Scientists also needed to isolate the virus’s spike protein from the bounty of genetic data provided by Chinese researchers. Dr. Barney Graham, of the National Institutes of Health, and Jason McClellan, of the University of Texas at Austin, solved that problem in short order.

Testing the quickly designed vaccines required a monumental effort by companies and the National Institutes of Health. But Dr. Kariko had no doubts.

On Nov. 8, the first results of the Pfizer-BioNTech study came in, showing that the mRNA vaccine offered powerful immunity to the new virus. Dr. Kariko turned to her husband. “Oh, it works,” she said. “I thought so.”

To celebrate, she ate an entire box of Goobers chocolate-covered peanuts. By herself.

Dr. Weissman celebrated with his family, ordering takeout dinner from an Italian restaurant, “with wine,” he said. Deep down, he was awed.

“My dream was always that we develop something in the lab that helps people,” Dr. Weissman said. “I’ve satisfied my life’s dream.”

Dr. Kariko and Dr. Weissman were vaccinated on Dec. 18 at the University of Pennsylvania. Their inoculations turned into a press event, and as the cameras flashed, she began to feel uncharacteristically overwhelmed.

A senior administrator told the doctors and nurses rolling up their sleeves for shots that the scientists whose research made the vaccine possible were present, and they all clapped. Dr. Kariko wept.

Things could have gone so differently, for the scientists and for the world, Dr. Langer said. “There are probably many people like her who failed,” he said.

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Researchers Are Hatching a Low-Cost Covid-19 Vaccine

A new vaccine for Covid-19 that is entering clinical trials in Brazil, Mexico, Thailand and Vietnam could change how the world fights the pandemic. The vaccine, called NVD-HXP-S, is the first in clinical trials to use a new molecular design that is widely expected to create more potent antibodies than the current generation of vaccines. And the new vaccine could be far easier to make.

Existing vaccines from companies like Pfizer and Johnson & Johnson must be produced in specialized factories using hard-to-acquire ingredients. In contrast, the new vaccine can be mass-produced in chicken eggs — the same eggs that produce billions of influenza vaccines every year in factories around the world.

If NVD-HXP-S proves safe and effective, flu vaccine manufacturers could potentially produce well over a billion doses of it a year. Low- and middle-income countries currently struggling to obtain vaccines from wealthier countries may be able to make NVD-HXP-S for themselves or acquire it at low cost from neighbors.

“That’s staggering — it would be a game-changer,” said Andrea Taylor, assistant director of the Duke Global Health Innovation Center.

Vaccines work by acquainting the immune system with a virus well enough to prompt a defense against it. Some vaccines contain entire viruses that have been killed; others contain just a single protein from the virus. Still others contain genetic instructions that our cells can use to make the viral protein.

Once exposed to a virus, or part of it, the immune system can learn to make antibodies that attack it. Immune cells can also learn to recognize infected cells and destroy them.

spike, latches onto cells and then allows the virus to fuse to them.

But simply injecting coronavirus spike proteins into people is not the best way to vaccinate them. That’s because spike proteins sometimes assume the wrong shape, and prompt the immune system to make the wrong antibodies.

The researchers injected the 2P spikes into mice and found that the animals could easily fight off infections of the MERS coronavirus.

The team filed a patent for its modified spike, but the world took little notice of the invention. MERS, although deadly, is not very contagious and proved to be a relatively minor threat; fewer than 1,000 people have died of MERS since it first emerged in humans.

But in late 2019 a new coronavirus, SARS-CoV-2, emerged and began ravaging the world. Dr. McLellan and his colleagues swung into action, designing a 2P spike unique to SARS-CoV-2. In a matter of days, Moderna used that information to design a vaccine for Covid-19; it contained a genetic molecule called RNA with the instructions for making the 2P spike.

Other companies soon followed suit, adopting 2P spikes for their own vaccine designs and starting clinical trials. All three of the vaccines that have been authorized so far in the United States — from Johnson & Johnson, Moderna and Pfizer-BioNTech — use the 2P spike.

Other vaccine makers are using it as well. Novavax has had strong results with the 2P spike in clinical trials and is expected to apply to the Food and Drug Administration for emergency use authorization in the next few weeks. Sanofi is also testing a 2P spike vaccine and expects to finish clinical trials later this year.

Dr. McLellan’s ability to find lifesaving clues in the structure of proteins has earned him deep admiration in the vaccine world. “This guy is a genius,” said Harry Kleanthous, a senior program officer at the Bill & Melinda Gates Foundation. “He should be proud of this huge thing he’s done for humanity.”

But once Dr. McLellan and his colleagues handed off the 2P spike to vaccine makers, he turned back to the protein for a closer look. If swapping just two prolines improved a vaccine, surely additional tweaks could improve it even more.

HexaPro, in honor of its total of six prolines.

The structure of HexaPro was even more stable than 2P, the team found. It was also resilient, better able to withstand heat and damaging chemicals. Dr. McLellan hoped that its rugged design would make it potent in a vaccine.

Dr. McLellan also hoped that HexaPro-based vaccines would reach more of the world — especially low- and middle-income countries, which so far have received only a fraction of the total distribution of first-wave vaccines.

“The share of the vaccines they’ve received so far is terrible,” Dr. McLellan said.

To that end, the University of Texas set up a licensing arrangement for HexaPro that allows companies and labs in 80 low- and middle-income countries to use the protein in their vaccines without paying royalties.

Meanwhile, Dr. Innes and his colleagues at PATH were looking for a way to increase the production of Covid-19 vaccines. They wanted a vaccine that less wealthy nations could make on their own.

experimenting with Newcastle disease virus to create vaccines for a range of diseases. To develop an Ebola vaccine, for example, researchers added an Ebola gene to the Newcastle disease virus’s own set of genes.

The scientists then inserted the engineered virus into chicken eggs. Because it is a bird virus, it multiplied quickly in the eggs. The researchers ended up with Newcastle disease viruses coated with Ebola proteins.

At Mount Sinai, the researchers set out to do the same thing, using coronavirus spike proteins instead of Ebola proteins. When they learned about Dr. McLellan’s new HexaPro version, they added that to the Newcastle disease viruses. The viruses bristled with spike proteins, many of which had the desired prefusion shape. In a nod to both the Newcastle disease virus and the HexaPro spike, they called it NDV-HXP-S.

announced the start of a clinical trial of NDV-HXP-S. A week later, Thailand’s Government Pharmaceutical Organization followed suit. On March 26, Brazil’s Butantan Institute said it would ask for authorization to begin its own clinical trials of NDV-HXP-S.

Meanwhile, the Mount Sinai team has also licensed the vaccine to the Mexican vaccine maker Avi-Mex as an intranasal spray. The company will start clinical trials to see if the vaccine is even more potent in that form.

To the nations involved, the prospect of making the vaccines entirely on their own was appealing. “This vaccine production is produced by Thai people for Thai people,” Thailand’s health minister, Anutin Charnvirakul, said at the announcement in Bangkok.

In Brazil, the Butantan Institute trumpeted its version of NDV-HXP-S as “the Brazilian vaccine,” one that would be “produced entirely in Brazil, without depending on imports.”

Ms. Taylor, of the Duke Global Health Innovation Center, was sympathetic. “I could understand why that would really be such an attractive prospect,” she said. “They’ve been at the mercy of global supply chains.”

Madhavi Sunder, an expert on intellectual property at Georgetown Law School, cautioned that NDV-HXP-S would not immediately help countries like Brazil as they grappled with the current wave of Covid-19 infections. “We’re not talking 16 billion doses in 2020,” she said.

Instead, the strategy will be important for long-term vaccine production — not just for Covid-19 but for other pandemics that may come in the future. “It sounds super promising,” she said.

In the meantime, Dr. McLellan has returned to the molecular drawing board to try to make a third version of their spike that is even better than HexaPro.

“There’s really no end to this process,” he said. “The number of permutations is almost infinite. At some point, you’d have to say, ‘This is the next generation.’”

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Researchers Are Hatching a Low-Cost Coronavirus Vaccine

A new vaccine for Covid-19 that is entering clinical trials in Brazil, Mexico, Thailand and Vietnam could change how the world fights the pandemic. The vaccine, called NVD-HXP-S, is the first in clinical trials to use a new molecular design that is widely expected to create more potent antibodies than the current generation of vaccines. And the new vaccine could be far easier to make.

Existing vaccines from companies like Pfizer and Johnson & Johnson must be produced in specialized factories using hard-to-acquire ingredients. In contrast, the new vaccine can be mass-produced in chicken eggs — the same eggs that produce billions of influenza vaccines every year in factories around the world.

If NVD-HXP-S proves safe and effective, flu vaccine manufacturers could potentially produce well over a billion doses of it a year. Low- and middle-income countries currently struggling to obtain vaccines from wealthier countries may be able to make NVD-HXP-S for themselves or acquire it at low cost from neighbors.

“That’s staggering — it would be a game-changer,” said Andrea Taylor, assistant director of the Duke Global Health Innovation Center.

Vaccines work by acquainting the immune system with a virus well enough to prompt a defense against it. Some vaccines contain entire viruses that have been killed; others contain just a single protein from the virus. Still others contain genetic instructions that our cells can use to make the viral protein.

Once exposed to a virus, or part of it, the immune system can learn to make antibodies that attack it. Immune cells can also learn to recognize infected cells and destroy them.

spike, latches onto cells and then allows the virus to fuse to them.

But simply injecting coronavirus spike proteins into people is not the best way to vaccinate them. That’s because spike proteins sometimes assume the wrong shape, and prompt the immune system to make the wrong antibodies.

The researchers injected the 2P spikes into mice and found that the animals could easily fight off infections of the MERS coronavirus.

The team filed a patent for its modified spike, but the world took little notice of the invention. MERS, although deadly, is not very contagious and proved to be a relatively minor threat; fewer than 1,000 people have died of MERS since it first emerged in humans.

But in late 2019 a new coronavirus, SARS-CoV-2, emerged and began ravaging the world. Dr. McLellan and his colleagues swung into action, designing a 2P spike unique to SARS-CoV-2. In a matter of days, Moderna used that information to design a vaccine for Covid-19; it contained a genetic molecule called RNA with the instructions for making the 2P spike.

Other companies soon followed suit, adopting 2P spikes for their own vaccine designs and starting clinical trials. All three of the vaccines that have been authorized so far in the United States — from Johnson & Johnson, Moderna and Pfizer-BioNTech — use the 2P spike.

Other vaccine makers are using it as well. Novavax has had strong results with the 2P spike in clinical trials and is expected to apply to the Food and Drug Administration for emergency use authorization in the next few weeks. Sanofi is also testing a 2P spike vaccine and expects to finish clinical trials later this year.

Dr. McLellan’s ability to find lifesaving clues in the structure of proteins has earned him deep admiration in the vaccine world. “This guy is a genius,” said Harry Kleanthous, a senior program officer at the Bill & Melinda Gates Foundation. “He should be proud of this huge thing he’s done for humanity.”

But once Dr. McLellan and his colleagues handed off the 2P spike to vaccine makers, he turned back to the protein for a closer look. If swapping just two prolines improved a vaccine, surely additional tweaks could improve it even more.

HexaPro, in honor of its total of six prolines.

The structure of HexaPro was even more stable than 2P, the team found. It was also resilient, better able to withstand heat and damaging chemicals. Dr. McLellan hoped that its rugged design would make it potent in a vaccine.

Dr. McLellan also hoped that HexaPro-based vaccines would reach more of the world — especially low- and middle-income countries, which so far have received only a fraction of the total distribution of first-wave vaccines.

“The share of the vaccines they’ve received so far is terrible,” Dr. McLellan said.

To that end, the University of Texas set up a licensing arrangement for HexaPro that allows companies and labs in 80 low- and middle-income countries to use the protein in their vaccines without paying royalties.

Meanwhile, Dr. Innes and his colleagues at PATH were looking for a way to increase the production of Covid-19 vaccines. They wanted a vaccine that less wealthy nations could make on their own.

experimenting with Newcastle disease virus to create vaccines for a range of diseases. To develop an Ebola vaccine, for example, researchers added an Ebola gene to the Newcastle disease virus’s own set of genes.

The scientists then inserted the engineered virus into chicken eggs. Because it is a bird virus, it multiplied quickly in the eggs. The researchers ended up with Newcastle disease viruses coated with Ebola proteins.

At Mount Sinai, the researchers set out to do the same thing, using coronavirus spike proteins instead of Ebola proteins. When they learned about Dr. McLellan’s new HexaPro version, they added that to the Newcastle disease viruses. The viruses bristled with spike proteins, many of which had the desired prefusion shape. In a nod to both the Newcastle disease virus and the HexaPro spike, they called it NDV-HXP-S.

announced the start of a clinical trial of NDV-HXP-S. A week later, Thailand’s Government Pharmaceutical Organization followed suit. On March 26, Brazil’s Butantan Institute said it would ask for authorization to begin its own clinical trials of NDV-HXP-S.

Meanwhile, the Mount Sinai team has also licensed the vaccine to the Mexican vaccine maker Avi-Mex as an intranasal spray. The company will start clinical trials to see if the vaccine is even more potent in that form.

To the nations involved, the prospect of making the vaccines entirely on their own was appealing. “This vaccine production is produced by Thai people for Thai people,” Thailand’s health minister, Anutin Charnvirakul, said at the announcement in Bangkok.

In Brazil, the Butantan Institute trumpeted its version of NDV-HXP-S as “the Brazilian vaccine,” one that would be “produced entirely in Brazil, without depending on imports.”

Ms. Taylor, of the Duke Global Health Innovation Center, was sympathetic. “I could understand why that would really be such an attractive prospect,” she said. “They’ve been at the mercy of global supply chains.”

Madhavi Sunder, an expert on intellectual property at Georgetown Law School, cautioned that NDV-HXP-S would not immediately help countries like Brazil as they grappled with the current wave of Covid-19 infections. “We’re not talking 16 billion doses in 2020,” she said.

Instead, the strategy will be important for long-term vaccine production — not just for Covid-19 but for other pandemics that may come in the future. “It sounds super promising,” she said.

In the meantime, Dr. McLellan has returned to the molecular drawing board to try to make a third version of their spike that is even better than HexaPro.

“There’s really no end to this process,” he said. “The number of permutations is almost infinite. At some point, you’d have to say, ‘This is the next generation.’”

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Virus Variants Threaten to Draw Out the Pandemic, Scientists Say

For weeks, the mood in much of the United States has been buoyant. Cases, hospitalizations and deaths from the coronavirus have fallen steeply from their highs, and millions of people are being newly vaccinated every day. Restaurants, shops and schools have reopened. Some states, like Texas and Florida, have abandoned precautions altogether.

In measurable ways, Americans are winning the war against the coronavirus. Powerful vaccines and an accelerating rollout all but guarantee an eventual return to normalcy — to backyard barbecues, summer camps and sleepovers.

But it is increasingly clear that the next few months will be painful. So-called variants are spreading, carrying mutations that make the coronavirus both more contagious and in some cases more deadly.

Even as vaccines were authorized late last year, illuminating a path to the pandemic’s end, variants were trouncing Britain, South Africa and Brazil. New variants have continued to pop up — in California one week, in New York and Oregon the next. As they take root, these new versions of the coronavirus threaten to postpone an end to the pandemic.

rising exponentially in the United States.

Limited genetic testing has turned up more than 12,500 cases, many in Florida and Michigan. As of March 13, the variant accounted for about 27 percent of new cases nationwide, up from just 1 percent in early February.

pledged a “down payment” of $200 million to ramp up surveillance, an infusion intended to make it possible to analyze 25,000 patient samples each week for virus variants. It’s an ambitious goal: The country was sequencing just a few hundred samples each week in December, then scaling up to about 9,000 per week as of March 27.

Until recently, B.1.1.7’s rise was camouflaged by falling rates of infection over all, lulling Americans into a false sense of security and leading to prematurely relaxed restrictions, researchers say.

“The best way to think about B.1.1.7 and other variants is to treat them as separate epidemics,” said Sebastian Funk, a professor of infectious disease dynamics at the London School of Hygiene and Tropical Medicine. “We’re really kind of obscuring the view by adding them all up to give an overall number of cases.”

Other variants identified in South Africa and Brazil, as well as some virus versions first seen in the United States, have been slower to spread. But they, too, are worrisome, because they contain a mutation that diminishes the vaccines’ effectiveness. Just this week, an outbreak of P.1, the variant that crushed Brazil, forced a shutdown of the Whistler Blackcomb ski resort in British Columbia.

as fast as possible.

Infections are rising again, driven to an uncertain degree by B.1.1.7 and other variants. Earlier this week, Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention, pleaded with Americans to continue to practice masking and social distancing, saying she felt a sense of “impending doom.”

60 percent more contagious and 67 percent more deadly than the original form of the virus, according to the most recent estimates.

The variant is no different from the original in how it spreads, but infected people seem to carry more of the virus and for longer, said Katrina Lythgoe, an evolutionary biologist at the University of Oxford. “You’re more infectious for more days,” she said.

So contagious is B.1.1.7 that Britain succeeded in driving down infections only after nearly three months of strict stay-at-home orders, plus an aggressive vaccination program. Even so, cases fell much more slowly than they did during a similar lockdown in March and April.

three-quarters of new infections, some hospitals have had to move coronavirus patients to Belgium to free up beds. Roughly as many people are dying each day from Covid-19 in Europe as were this time a year ago.

For too long, government officials disregarded the threat. “Case plateaus can hide the emergence of new variants,” said Carl Pearson, a research fellow at the London School of Hygiene and Tropical Medicine. “And the higher those plateaus are, the worse the problem is.”

In the United States, coronavirus infections began a rapid decline in January, soon prompting many state leaders to reopen businesses and ease restrictions. But scientists repeatedly warned that the drop would not last. After the rate bottomed out at about 55,000 cases and 1,500 deaths per day in mid-March, some states — notably Michigan — began seeing an uptick.

Since then, the national numbers have steadily risen. As of Saturday, the daily count was up to nearly 69,000, and the weekly average was 19 percent higher than the figure two weeks earlier.

Pfizer-BioNTech and Moderna vaccines seem to be slightly less effective against B.1.351, the variant identified in South Africa. That variant contains the Eek mutation, which seems to enable the virus to partly sidestep the body’s immune response. The vaccines made by Johnson & Johnson, AstraZeneca and Novavax were even less potent against B.1.351.

“I think for the next year or two, E484K will be the most concerning” mutation, said Jesse Bloom, an evolutionary biologist at the Fred Hutchinson Cancer Research Center in Seattle.

The mutation slightly alters the so-called spike protein sitting on the surface of the coronavirus, making it just a bit harder for antibodies to latch on and destroy the invader.

The good news is that the virus seems to have just a few survival tricks in its bag, and that makes it easier for scientists to find and block those defenses. “I’m feeling pretty good about the fact that there aren’t that many choices,” said Michel Nussenzweig, an immunologist at Rockefeller University in New York.

The Eek mutation seems to be the virus’s primary defense against the immune system. Researchers in South Africa recently reported that a new vaccine directed against B.1.351 ought to fend off all other variants, as well.

Pfizer, BioNTech and Moderna already are testing newly designed booster shots against B.1.351 that should work against any variants known to blunt the immune response.

Instead of a new vaccine against variants, however, it may be just as effective for Americans to receive a third dose of the Pfizer-BioNtech or Moderna vaccines in six months to a year, said Dr. Anthony S. Fauci, head of the National Institute of Allergy and Infectious Diseases.

That would keep antibody levels high in each recipient, overwhelming any variant — a more practical strategy than making a specialized vaccine for each new variant that emerges, he said.

“My only concern about chasing all the variants is that you’d almost be playing Whac-A-Mole, you know, because they’ll keep coming up and keep coming up,” Dr. Fauci said.

In one form or another, the new coronavirus is here to stay, many scientists believe. Multiple variants may be circulating in the country at the same time, as is the case for common cold coronaviruses and influenza. Keeping them at bay may require an annual shot, like the flu vaccine.

The best way to deter the emergence of dangerous variants is to keep cases down now and to immunize the vast majority of the world — not just the United States — as quickly as possible. If significant pockets of the globe remain unprotected, the virus will continue to evolve in dangerous new ways.

“This might be something that we have to deal with for a long time,” said Rosalind Eggo, an epidemiologist at London School of Hygiene and Tropical Medicine.

Still, she added, “Even if it changes again, which it is very likely to do, we are in a better, much stronger position than a year ago to deal with it.”

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