In Arkansas and Illinois, health officials have directed teams to call, text or send letters to people to remind them to get their second shots. In Pennsylvania, officials are trying to ensure that college students can get their second shots after they leave campus for the summer. South Carolina has allocated several thousand doses specifically for people who are overdue for their second shot.

recommended restarting use of the vaccine, but the combination of the safety scare and ongoing production problems is likely to make that vaccine a viable option for fewer people.

The C.D.C.’s count of missed second doses is through April 9. It covers only people who got a first Moderna dose by March 7 or a first Pfizer dose by March 14.

Roughly three-quarters of adults come back for their second dose of the vaccine that protects against shingles.

In some cases, problems with shipments or scheduling may be playing a role in people missing their second doses. Some vaccine providers have had to cancel appointments because they did not receive expected vaccine deliveries. People have also reported having their second-dose appointments canceled or showing up only to find out that there were no doses available of the brand they needed.

Some people can be flexible about being rebooked. But that’s harder for people who lack access to reliable transportation or who have jobs with strictly scheduled hours, said Elena Cyrus, an infectious disease epidemiologist at the University of Central Florida.

Walgreens booked some customers for their second appointments at places that didn’t have the same vaccine that they had received for their initial doses. The company said it fixed the problem in late March.

come under fire for, until recently, scheduling second doses of the Pfizer vaccine four weeks after the first shot, rather than the three-week gap recommended by the C.D.C. Pharmacists have been besieged by customers complaining, including about their inability to book vaccine appointments online.

In other cases, though, access to vaccines is not the sole barrier; people’s attitudes contribute, too.

Basith Syed, a 24-year-old consultant in Chicago, nabbed a leftover Moderna vaccine at a Walgreens in mid-February. But when the time came for his second shot, he was busy at work and preparing for his wedding. After the first shot, he had spent two days feeling drained. He didn’t want to risk a repeat, and he felt confident that a single dose would protect him.

C.D.C. says there is limited data on the vaccine’s effectiveness when shots are separated by more than six weeks, although some countries, including Britain and Canada, are giving shots with a gap of up to three or four months.

Mr. Syed’s experience is part of a broader shift in Illinois. When vaccines were mostly being given to health care workers, residents of long-term care facilities and people over 65, almost everyone was getting their second shots. In recent weeks, though, the number dipped below 90 percent, though it has since rebounded slightly, according to the Illinois Department of Public Health.

In Arkansas, about 84,000 people have missed their second shots, representing 11 percent of those eligible for those shots, said Dr. Jennifer Dillaha, the state epidemiologist. Workers recently began calling people who are due or overdue for their second shots.

College students pose a particular challenge. Many recently became eligible to be vaccinated and are getting their first shots, but they will have left campus by the time they are due for their second doses.

In Pennsylvania, health officials have instructed vaccine providers to give second doses to college students even if they did not receive their first doses from that location.

Some vaccine providers have put on special clinics for people who need a second dose. In South Carolina, the health system Tidelands Health started a program specifically for people who received their first Pfizer doses more than 23 days earlier but hadn’t been able to find a second shot. The state health department sent the health system 2,340 doses for the effort.

Demand has been strong, and Tidelands only has a few hundred doses left. The majority of takers have been people who “were having difficulty navigating all the various scheduling systems and providers,” said Gayle Resetar, the health system’s chief operating officer.

In many cases, vaccine providers had canceled second-dose appointments because of bad winter weather. “It was up to the individual to reschedule themselves on a web portal or web platform, and that just became difficult for people,” Ms. Resetar said.

There are rare cases in which people are supposed to forgo the second shot, such as if they had an allergic reaction after their first shot.

Zvi Ish-Shalom, a religious studies professor from Boulder, Colo., had planned to get fully vaccinated. Then, an hour after his first shot of the Moderna vaccine, he developed a headache that hasn’t gone away more than a month later.

There is no way to know for sure whether the vaccine triggered the headache. But after weighing what he saw as the risks and benefits of a second dose, Dr. Ish-Shalom reached a decision about how to proceed.

“At this point in time, I feel very clear and very comfortable, given all the various elements of this equation, to forgo the second shot,” he said.

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Vaccines Are Effective Against the New York Variant, Studies Find

For weeks, New Yorkers have witnessed the alarming rise of a homegrown variant of the coronavirus that has kept the number of cases in the city stubbornly high. City officials have repeatedly warned that the variant may be more contagious and may dodge the immune response.

On that second point, at least, they can now breathe easier: Both the Pfizer-BioNTech and Moderna vaccines will effectively prevent serious illness and death from the variant, two independent studies suggest.

Antibodies stimulated by those vaccines are only slightly less potent at controlling the variant than the original form of the virus, both studies found.

“We’re not seeing big differences,” said Michel Nussenzweig, an immunologist at Rockefeller University in New York and a member of the team that published one of the studies on Thursday.

possible explanation: Antibodies from vaccinated people are distributed across a broader range of parts of the virus, so no single mutation has a big impact on their effectiveness — making vaccines a better bet against variants than immunity from natural infection.

The variant first identified in New York, known to scientists as B.1.526, raced through the city after its initial discovery in November. It accounted for one in four diagnosed cases by November and nearly half of cases as of April 13. The variant that brought Britain to a standstill, B.1.1.7, is also circulating widely in New York. Together, the two add up to more than 70 percent of coronavirus cases in the city.

second study, Dr. Landau’s team found that the Pfizer and Moderna vaccines are only marginally less protective against the variant that devastated Britain and against forms of the variant discovered in New York that don’t contain the Eek mutation.

Several laboratory studies have shown that antibodies induced by the Pfizer and Moderna vaccines are slightly less powerful against a third variant, one identified in South Africa, which also contains Eek. Other vaccines fared worse. South Africa suspended use of the AstraZeneca vaccine after clinical trials showed that the vaccine did not prevent mild or moderate illness from the variant that was circulating there.

“It already started out as a lower level in terms of the immunity that it generated,” Dr. Nussenzweig said of the AstraZeneca vaccine. Referring to the Pfizer and Moderna shots, he said, “We’re so lucky in this country to have these vaccines compared to the rest of the world.”

Florian Krammer, an immunologist at the Icahn School of Medicine at Mount Sinai who was not involved in either of the new studies, said he was more concerned about other countries’ vaccine programs than about the variants themselves.

“I am less concerned about variants than I was two months ago,” he said, but added: “I’m worried about countries that don’t have enough vaccine and that don’t have that vaccine rollout. I’m not worried anymore about the U.S., honestly.”

Dr. Landau’s team also tested monoclonal antibodies used to treat Covid-19 against the variants. They found that the cocktail of monoclonal antibodies made by Regeneron worked as well against the variant discovered in New York as against the original virus.

The studies are reassuring, but they indicate that the Eek mutation is one to watch, said Jesse Bloom, an evolutionary biologist at the Fred Hutchinson Cancer Research Center in Seattle.

“This could certainly be a step toward the virus becoming somewhat more resistant to infection- and vaccine-mediated immunity,” Dr. Bloom said. “I don’t think it’s something that people need to immediately become alarmed about, but it definitely impresses us as important.”

Dr. Bloom led the analysis comparing vaccine-induced antibodies with those produced by natural infection. He found that the most powerful antibodies bind to multiple sites in a key part of the virus. Even if a mutation affects the binding in one site in this region, antibodies that target the remaining sites would still be protective.

Antibodies induced by the vaccine cover many more sites across this region than those from natural infection — and so are less likely to be affected by a mutation in any one site.

The study looked only at antibodies stimulated by the Moderna vaccine, but the results are likely to be the same for the Pfizer-BioNTech vaccine, he added.

“This could potentially be a good thing as the virus is creating mutations,” Dr. Bloom said.

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A Week of Discouraging Coronavirus Pandemic Developments in Canada

I was feeling vaguely guilty this week when heading out to a sports complex in suburban Ottawa for my vaccination. As I write this, only 19 percent of Canadians have shared my experience and just before my vaccine day arrived, tens of thousands of vaccination appointments in Manitoba and Ontario were canceled.

pulled slightly ahead of the United States in average daily new cases per capita. Moderna cut deliveries of its vaccine to Canada and other countries while the Johnson & Johnson vaccine, which has yet to arrive in Canada, has come under safety scrutiny.

paused the use of Johnson & Johnson’s one-dose vaccine over concerns that it might be linked to a rare but serious blood-clotting disorder. Canada is expecting its first shipment of that vaccine — 300,000 doses — on April 27.

My colleagues Denise Grady and Carl Zimmer examined the blood-clotting risk potentially posed by that vaccine as well as the AstraZeneca vaccine. Their bottom line: If there is a risk, it’s low.

[Read: J & J Vaccine and Blood Clots: The Risks, if Any, Are Very Low]

But perhaps offsetting all that is Mr. Trudeau’s announcement that Pfizer will sell Canada an additional eight million doses of the vaccine it has developed with BioNTech, half of which will arrive next month, and all of which will arrive by the end of July. The company will also be sending earlier purchases sooner. All that may mean that all Canadian adults will have received at least one shot by July, the prime minister said.

an immediate pause in the use of Johnson & Johnson’s single-dose Covid-19 vaccine after six recipients in the United States developed a rare disorder involving blood clots within one to three weeks of vaccination.

  • All 50 states, Washington, D.C. and Puerto Rico temporarily halted or recommended providers pause the use of the vaccine. The U.S. military, federally run vaccination sites and a host of private companies, including CVS, Walgreens, Rite Aid, Walmart and Publix, also paused the injections.
  • Fewer than one in a million Johnson & Johnson vaccinations are now under investigation. If there is indeed a risk of blood clots from the vaccine — which has yet to be determined — that risk is extremely low. The risk of getting Covid-19 in the United States is far higher.
  • The pause could 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.
  • Johnson & Johnson has also decided to delay the rollout of its vaccine in Europe amid concerns over rare blood clots, dealing another blow to Europe’s inoculation push. South Africa, devastated by a more contagious virus variant that emerged there, suspended use of the vaccine as well. Australia announced it would not purchase any doses.
  • As I pulled into a parking lot, a man in an orange vest told me to stay in the car until my appointment time was announced over a very loud loudspeaker to avoid people congregating. After passing through two screenings by people who remained welcoming, despite having to endlessly ask the same questions, and a registration check in, I received a shot four minutes after my scheduled appointment time. It was injected by someone more than qualified for the task: an orthopedic surgeon.

    Canada’s decision to get at least one shot into as many people as possible means that I’m not scheduled for a second dose until August.

    As many Canadians look at vaccination rates in Britain and the United States, their frustration has been growing. Right now, just 2 percent of Canadians are fully vaccinated compared with 24 percent of Americans. But the scheduled increases in vaccine shipments — the Moderna slip up aside — should help Canada catch up slightly over the next few weeks.

    If so, it will also be a relief to the medical world. After he released the projections compiled by Ontario’s table of science experts on Friday, which indicated cases could hit 30,000 a day if nothing is done, Adalsteinn Brown, the dean of the Dalla Lana School of Public Health at the University of Toronto, said, “More vaccination, more vaccination, more vaccination.”


    built 100 tiny shelters for homeless people to get though the winter. He now has an even bigger plan.

  • Geneva Abdul, a Times colleague now based in London and former member of Canada’s national soccer team, wrote about the confidence that playing the sport gave her.

  • An exhaustive review found that anti-gay bias by Toronto police helped allow a serial killer to prey on the city’s gay community.

  • William Amos, a Liberal member of Parliament from Quebec, stripped down after a jog while not realizing that his computer’s camera was on and broadcasting to his fellow lawmakers in a virtual meeting. Now some people are asking who leaked the photo of Mr. Amos standing nude to the public.


  • A native of Windsor, Ontario, Ian Austen was educated in Toronto, lives in Ottawa and has reported about Canada for The New York Times for the past 16 years. Follow him on Twitter at @ianrausten.


    How are we doing?
    We’re eager to have your thoughts about this newsletter and events in Canada in general. Please send them to nytcanada@nytimes.com.

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    Moderna Says Vaccine Protects Against Covid After Six Months

    The Moderna coronavirus vaccine continues to provide strong protection in the United States against Covid-19 six months after vaccination, the company announced Tuesday.

    The vaccine, after both doses are given, has been more than 90 percent effective at protecting against Covid-19 and 95 percent effective at protecting against severe disease, the company said in a statement. The results are based on more than 900 total cases of Covid-19, including 100 severe cases, that have been identified in participants of Moderna’s phase three vaccine trial as of April 9.

    Pfizer recently announced that its vaccine was more than 90 percent effective against Covid-19 after six months and 95 to 100 percent effective against severe disease. Both companies are now testing their vaccines in children.

    It is not yet clear whether the overall effectiveness of the vaccines will drop as dangerous new variants, some of which may evade vaccine-induced antibodies, spread more widely throughout the United States.

    consistent with results that were published in The New England Journal of Medicine last week. In that report, which is based on 33 adults who had received the Moderna vaccine, researchers found that study participants continued to have high levels of antibodies against the coronavirus six months after their second dose.

    The company also announced preliminary results from tests of potential vaccine boosters in mice. One shot is designed to protect against B.1.351, the variant first identified in South Africa. The other combines that more targeted vaccine with the original formulation. Both boosters increased antibody levels in the mice, but the latter seemed to produce the strongest immune response in the mice, the company said, though it did not provide more detailed data.

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    Worry Over 2 Covid Vaccines Deals Fresh Blow to Europe’s Inoculation Push

    BRUSSELS — First it was AstraZeneca. Now Johnson & Johnson.

    Last week, British regulators and the European Union’s medical agency said they had established a possible link between AstraZeneca’s Covid-19 vaccine and very rare, though sometimes fatal, blood clots.

    On Tuesday, Johnson & Johnson said it would pause the rollout of its vaccine in Europe and the United States over similar concerns, further compounding the continent’s one-step-forward-two-steps-back efforts to quickly get people immunized against the coronavirus.

    European officials had been confident that they had secured enough alternative vaccine doses to take up the slack of the AstraZeneca problems and achieve their goal of fully inoculating 70 percent of the European Union’s adult population — about 255 million people — by the end of the summer.

    On Tuesday, European officials did not immediately say whether they believed the milestone would also survive the Johnson & Johnson suspension. But the European commissioner for health, Stella Kyriakides, wrote on Twitter that “Today’s developments with the J&J vaccine in the US are under close monitoring” by the bloc’s medicines regulator.

    months of short supplies and logistical problems.

    There is mounting evidence that the concerns are eroding Europeans’ willingness to get the AstraZeneca vaccine in particular, and threatening to elevate already high levels of vaccine hesitancy generally.

    YouGov poll published last month, 61 percent of the French, 55 percent of Germans and 52 percent of Spaniards consider the AstraZeneca vaccine “unsafe.” That is in stark contrast to the findings of a similar poll from February, when more people in those countries, with the exception of France, believed that the shot was more safe than unsafe.

    Regulators have asked vaccine recipients and doctors to be on the lookout for certain symptoms, including severe and persistent headaches and tiny blood spots under the skin. Doctors’ groups have circulated guidance about how to treat the disorder.

    In Poland, where the vaccination campaign relies to a large extent on AstraZeneca and where its use has not been restricted, a recent poll showed that given a choice, fewer than 5 percent of Poles would choose the AstraZeneca shot.

    Almost everywhere across the European Union, it seems, many are eager for alternatives, as the new types of vaccines that include the Moderna and Pfizer, which utilize science known as “mRNA,” have not been associated with similar side effects.

    Data from the 27 E.U. member states by the European Center for Disease Prevention and Control shows that over all, 80 percent of vaccine doses distributed to the bloc have already been administered. That share drops to 65 percent for AstraZeneca, however, suggesting that many of its doses are sitting unused.

    Yet it is hard to predict how serious a blow the latest twist in the AstraZeneca saga — and the new Johnson & Johnson concerns — will be to E.U. vaccination efforts, as officials in Brussels have made big if belated efforts to turbocharge the second-quarter supply of doses.

    The European Union is poised to receive at least 300 million doses of various vaccines, three times what it got in the first quarter. Two hundred million are slated to come from Pfizer/BioNTech. Moderna is expected to deliver 35 million doses. Another 55 million doses are due of the Johnson & Johnson jab, and 70 million from AstraZeneca.

    In the rosiest scenario, the European Union could get up to 360 million doses by June.

    On Thursday, after Spain’s government changed the age threshold for the AstraZeneca shot, two-thirds of people called up for vaccination in Madrid did not show up, Antonio Zapatero, the regional health minister, told a news conference on Friday.

    He attributed the no-show by 18,200 people to “confusion” generated by Spain’s central government, which said on Wednesday that the AstraZeneca vaccine should be given only to people over 60. Before this change, Mr. Zapatero said, the rate of abstention was 2 percent.

    In Belgium, where the use of the AstraZeneca vaccine has also been limited, the authorities said they did not expect major delays in the overall rollout, but they are still concerned about the confusion that the rare blood clotting issue is causing.

    Yves Van Laethem, a top epidemiologist who is the country’s Covid task force spokesman, said he expected a two-week delay that would mostly affect younger age groups in late summer. He said the E.U. regulator guidance had only partly helped in clarifying the situation.

    The European Medicines Agency’s opinion “wasn’t very clear, and it is also part of the problem,” Dr. Van Laethem said in an interview. “When you say, ‘We don’t apply limitations, but we just say there are serious side effects,’ there is part science and part diplomacy in that.”

    He said the limited effect that the new AstraZeneca issues would have on Belgian’s rollout was in large part because the country had ordered big shares of other vaccines.

    Although all E.U. countries have been offered a chunk of each vaccine approved in the bloc so far — AstraZeneca, Johnson & Johnson, Moderna and Pfizer — many opted to forgo parts of their share of more expensive or cumbersome vaccines like Pfizer and Moderna early on, instead favoring the AstraZeneca jab.

    “In Britain or Eastern Europe, a big part of the campaigns are based on AstraZeneca,” Dr. Van Laethem said.

    Wealthier bloc members like Denmark, France, Germany and the Netherlands can better compensate for the loss of confidence in AstraZeneca, because they acquired extra doses of other vaccines — especially Pfizer — through a secondary market after poorer E.U. nations gave theirs up.

    Those countries — including Bulgaria, Croatia, Latvia and Slovakia — are likely to be less able to quickly offer alternatives.

    Dr. Van Laethem, the Belgian immunologist, said that the national and European authorities needed to better communicate the costs and benefits of taking the AstraZeneca dose versus and the other authorized vaccines.

    Experts worry that even limited concerns over one vaccine’s unlikely side effects can affect people’s overall willingness to be immunized.

    “The main thing is to make people understand that the problem is the virus,” he said. “We have to vaccinate people — the risk linked to the virus is higher than those rare side effects.”

    Raphael Minder contributed reporting from Madrid and Constant Méheut from Paris.

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    China Official Acknowledges Low Effectiveness of the Country’s Covid Vaccines

    A senior Chinese official said that the country’s vaccines may need to be administered in greater doses or in concert with other shots because of their low overall effectiveness.

    The comments on Saturday by Gao Fu, the director of China’s disease control center, suggest that China and more than 60 countries that have approved Chinese vaccines could need to adjust their distribution programs. The widespread distribution of Chinese vaccines means that any changes could potentially affect hundreds of millions of people or more.

    Possible steps to boost effectiveness of Chinese vaccines include changing the amount of vaccine given, the number of shots, the time between shots or the type of vaccines given, Mr. Gao said.

    He also praised the possibilities offered by messenger RNA. That technology is used in the Moderna and Pfizer-BioNTech vaccines but not in any of the vaccines thus far approved in China.

    said in January that the efficacy rate for the CoronaVac vaccine from the Beijing-based company Sinovac was just over 50 percent. By comparison, Moderna and Pfizer-BioNTech were found to be 90 percent effective in real world conditions, researchers said last month.

    Last month the distributor in the United Arab Emirates of vaccines from China’s Sinopharm said it was offering a third dose in addition to the standard two-dose regimen for a “very small number” of people who were “not really responsive” to the vaccine.

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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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    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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    The Pfizer-BioNTech Vaccine Is Said to Be Powerfully Protective in Adolescents

    The Pfizer-BioNTech coronavirus vaccine is extremely effective in adolescents 12 to 15 years old, perhaps even more so than in adults, the companies reported on Wednesday. No infections were found among children who received the vaccine in a recent clinical trial, the drug makers said; the children produced strong antibody responses and experienced no serious side effects.

    The findings, if they hold up, may speed a return to normalcy for millions of American families. Depending on regulatory approval, vaccinations could begin before the start of the next academic year for middle school and high school students, and for elementary school children not long after.

    The companies announced the results in a news release that did not include detailed data from the trial, which has not yet been peer-reviewed nor published in a scientific journal. Still, the news drew praise and excitement from experts.

    “Oh my god, I’m so happy to see this — this is amazing,” said Akiko Iwasaki, an immunologist at Yale University. If the vaccines’s performance in adults was A-plus, the results in children were “A-plus-plus.”

    had left her with sense of “impending doom,” while President Biden called on state and local officials to reinstate mask mandates.

    Vaccination efforts are accelerating throughout the nation. As of Tuesday, 29 percent of adults had received at least one dose of a coronavirus vaccine, and 16 percent had been fully inoculated, according to the C.D.C.

    But the country cannot hope to reach herd immunity — the point at which immunity becomes so widespread that the coronavirus slows its crawl through the population — without also inoculating the youngest Americans, some experts say. Children under 18 account for about 23 percent of the population in the United States.

    “The sooner that we can get vaccines into as many people as possible, regardless of their age, the sooner we will be able to really feel like we’re ending this pandemic for good,” said Angela Rasmussen, a virologist affiliated with Georgetown University in Washington.

    Data from Israel suggest that vaccinating adults alone can significantly decrease the number of cases, but “long term, to hit the herd immunity threshold, we will have to vaccinate children,” she said.

    children ages 5 to 11 just last week. Company scientists plan to start testing the vaccine next week in even younger children, ages 2 to 5, followed by trials in children ages 6 months to 2 years.

    testing its vaccine in children. Results from a trial in adolescents ages 12 to 17 are expected in the next few weeks and in children 6 months to 12 years old in the second half of this year.

    AstraZeneca started testing its vaccine in children 6 months and older last month, and Johnson & Johnson has said it will wait for results from trials in older children before testing its vaccine in children under 12.

    Some parents have said they are reluctant to immunize their children because the risk posed by the virus is low. Children make up fewer than 1 percent of deaths from Covid-19, but about 2 percent of children who get the illness require hospital care.

    The new results may not sway all of those parents, but they may reassure parents who have been wary of the vaccines, said Jennifer Nuzzo, an epidemiologist at the Johns Hopkins Center for Health Security.

    “While I don’t think we have to wait until children are vaccinated to fully reopen schools, being able to vaccinate children may help some families feel safer about returning to school,” she said.

    Pfizer and BioNTech plan to request from the Food and Drug Administration an amendment to the emergency use authorization for their vaccine, in hopes of beginning immunizations of older children before the start of the next school year. The companies also are planning to submit their data for peer review and publication in a scientific journal.

    They will monitor the participants for two years after the second dose to assess the vaccine’s long-term safety and efficacy. Side effects of vaccines are usually apparent within the first six weeks, said Dr. Kristin Oliver, a pediatrician and vaccine expert at Mount Sinai Hospital in New York. “Still, it’s good to know that safety monitoring is going to continue,” she said.

    The C.D.C. recommends that people avoid getting other vaccines for two weeks before and after receiving the two doses of the coronavirus vaccine.

    But children receive more vaccines in the few weeks before the school year than at any other time, Dr. Oliver noted, so pediatricians and parents should aim to get those other immunizations done earlier than usual.

    The coronavirus vaccines should ideally be given by pediatricians who have deep experience in immunizing children, Dr. Oliver added. “Now is the time to start planning how that rollout is going to take place in this age group,” she said.

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    Moderna and Pfizer-BioNTech Vaccines Are Very Effective, Report Says

    The coronavirus vaccines made by Moderna and Pfizer-BioNTech are proving highly effective at preventing symptomatic and asymptomatic infections under real-world conditions, federal health researchers reported on Monday.

    Consistent with clinical trial data, a two-dose regimen prevented 90 percent of infections by two weeks after the second shot. One dose prevented 80 percent of infections by two weeks after vaccination.

    There has been debate over whether vaccinated people can still get asymptomatic infections and transmit the virus to others. The study, by researchers at the Centers for Disease Control and Prevention, suggested that transmission may be extremely unlikely, as infections were so rare.

    There also has been concern that variants may render the vaccines less effective. The study’s results do not confirm that fear. Troubling variants were circulating during the time of the study — from December 14, 2020 to March 13, 2021 — yet the vaccines still provided powerful protection.

    The C.D.C. enrolled 3,950 people at high risk of being exposed to the virus because they were health care workers, first responders, or others on the front lines. None had previously been infected with the coronavirus.

    Most — 62.8 percent — received both shots of the vaccine during the study period, and 12.1 percent had one shot. The participants had no previous infections with the coronavirus.

    Participants collected their own nasal swabs each week, which were sent to a central location for P.C.R. testing, the most accurate type of test. The weekly swabs allowed the researchers to detect asymptomatic infections as well as symptomatic ones.

    The investigators also asked participants about symptoms associated with infection, including fever, chills, cough, shortness of breath, sore throat, diarrhea, muscle aches, or loss of smell or taste. The researchers also analyzed patients’ medical records to detect illnesses.

    Fifty-eight percent of the infections were detected before people had symptoms. Just 10.2 percent of infected people never developed symptoms.

    Among those who were fully vaccinated, there were .04 infections per 1,000 person-days, meaning that among 1,000 persons there would be .04 infections in a day.

    There were 0.19 infections per 1,000 person-days among those who had had one dose of the vaccine. In contrast, there were 1.38 infections per 1,000 person-days in unvaccinated people.

    “This study shows that our national vaccination efforts are working.,” said Dr. Rochelle P. Walensky, the C.D.C. director, in a news release.

    The C.D.C. said this is the first of many vaccine effectiveness studies it will be conducting.

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