although the vaccine remains powerfully effective against severe disease and death.

Experts were divided on the utility of booster shots so soon after vaccination began. Experience with other diseases indicates that older people and those with weak immune systems might benefit, but there is little hard evidence with the coronavirus.

“The problem here is, we’re just sort of going on immunological priors, rather than really great data to justify things one way or the other,” said Deepta Bhattacharya, an immunologist at the University of Arizona. “I totally understand the decision, but I think we have to acknowledge that there’s a wide range of uncertainty on what it’s going to do.”

Booster doses may help some people with weak immune systems, but others may show little improvement even after a third dose, and still others may not need a booster at all, scientists say.

While dozens of mostly wealthy countries, including the United States and most of Europe, have administered more than 100 doses per 100 people, many other nations remain below five per 100 — primarily in Africa, where cases have soared as the Delta variant spreads.

Doctors Without Borders said recently that it would be “unconscionable” to give booster doses in richer nations before people in poorer ones get their first doses.

“Wealthy governments shouldn’t be prioritizing giving third doses when much of the developing world hasn’t even yet had the chance to get their first Covid-19 shots,” Kate Elder, the senior vaccines policy adviser at Doctors Without Borders’ Access Campaign, said in a statement.

a so-called vector vaccine, like AstraZeneca or Johnson & Johnson.

It is the latest sign that governments are encouraging their citizens to mix and match vaccines in the hope of provoking a more protective immune response against Covid-19. Early results from a British vaccine study showed that volunteers produced high levels of antibodies and immune cells after getting one dose each of the Pfizer-BioNTech and AstraZeneca-Oxford shots.

The new German guidelines announced Monday also went a step further in encouraging parents to vaccinate children between 12 and 17, announcing that doctors and vaccination centers across the country would make the jab available to them before the start of the new school year.

Health ministers stopped short of making a formal recommendation for vaccinating children, but the move made plain their impatience with Germany’s Standing Committee on Vaccinations, which has so far refrained from guiding parents one way or the other, pending more data becoming available.

Vaccinating children “is one building block to allow a safe start into the new school year after the summer vacation,” Mr. Holetschek said.

Apoorva Mandavilli contributed reporting from New York, Benjamin Mueller from London, Aurelien Breeden from Paris, Gaia Pianigiani from Rome, Monika Pronczuk from Brussels, Raphael Minder from Madrid and Thomas Erdbrink from Amsterdam.

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Immunity to the Coronavirus May Persist for Years, Scientists Find

Immunity to the coronavirus lasts at least a year, possibly a lifetime, improving over time especially after vaccination, according to two new studies. The findings may help put to rest lingering fears that protection against the virus will be short-lived.

Together, the studies suggest that most people who have recovered from Covid-19 and who were later immunized will not need boosters. Vaccinated people who were never infected most likely will need the shots, however, as will a minority who were infected but did not produce a robust immune response.

Both reports looked at people who had been exposed to the coronavirus about a year earlier. Cells that retain a memory of the virus persist in the bone marrow and may churn out antibodies whenever needed, according to one of the studies, published on Monday in the journal Nature.

The other study, which is also under review for publication in Nature, found that these so-called memory B cells continue to mature and strengthen for at least 12 months after the initial infection.

other studies.

Some scientists have interpreted this decrease as a sign of waning immunity, but it is exactly what’s expected, other experts said. If blood contained high quantities of antibodies to every pathogen the body had ever encountered, it would quickly transform into a thick sludge.

Instead, blood levels of antibodies fall sharply following acute infection, while memory B cells remain quiescent in the bone marrow, ready to take action when needed.

landmark study in 2007 showed that antibodies in theory could survive decades, perhaps even well beyond the average life span, hinting at the long-term presence of memory B cells. But the new study offered a rare proof of their existence, Dr. Gommerman said.

Dr. Nussenzweig’s team looked at how memory B cells mature over time. The researchers analyzed blood from 63 people who had recovered from Covid-19 about a year earlier. The vast majority of the participants had mild symptoms, and 26 had also received at least one dose of either the Moderna or the Pfizer-BioNTech vaccine.

So-called neutralizing antibodies, needed to prevent reinfection with the virus, remained unchanged between six and 12 months, while related but less important antibodies slowly disappeared, the team found.

confirming results from other studies; the shots also ramped up the body’s neutralizing ability by about 50-fold.

Senator Rand Paul, Republican of Kentucky, said on Sunday that he would not get a coronavirus vaccine because he had been infected in March of last year and was therefore immune.

But there is no guarantee that such immunity will be powerful enough to protect him for years, particularly given the emergence of variants of the coronavirus that can partially sidestep the body’s defenses.

The results of Dr. Nussenzweig’s study suggest that people who have recovered from Covid-19 and who have later been vaccinated will continue to have extremely high levels of protection against emerging variants, even without receiving a vaccine booster down the line.

“It kind of looks exactly like what we would hope a good memory B cell response would look like,” said Marion Pepper, an immunologist at the University of Washington in Seattle who was not involved in the new research.

The experts all agreed that immunity is likely to play out very differently in people who have never had Covid-19. Fighting a live virus is different from responding to a single viral protein introduced by a vaccine. And in those who had Covid-19, the initial immune response had time to mature over six to 12 months before being challenged by the vaccine.

“Those kinetics are different than someone who got immunized and then gets immunized again three weeks later,” Dr. Pepper said. “That’s not to say that they might not have as broad a response, but it could be very different.”

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Japan Approves Moderna and AstraZeneca Vaccines

Japan on Friday approved the Moderna and AstraZeneca coronavirus vaccines for use in adults, giving the country much-needed new options as it tries to speed up an inoculation campaign that has been one of the slowest in the developed world.

Previously, only the Pfizer vaccine had been authorized for use in Japan, where just 4.1 percent of the population has received a first shot. Vaccinations have been held up by strict rules that allow only doctors and nurses to administer shots, and by a requirement that vaccines be tested on people in Japan before they are approved for use.

Japan is in the midst of a fourth wave of Covid-19 infections, just two months before the Summer Olympics in Tokyo are set to begin. Tokyo and eight other prefectures are under a state of emergency that will last at least until the end of this month, and Okinawa is expected to be added to that list. Japan has been reporting about 5,500 cases a day, compared to 1,000 in early March.

A health ministry panel recommended on Thursday night that the government approve the Moderna and AstraZeneca vaccines. The health minister, Norihisa Tamura, said the Moderna shots would be used at mass inoculation sites scheduled to open on Monday in Tokyo and Osaka, which will be staffed mainly by military doctors and nurses.

very rare cases of blood clotting.

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Taiwan Faces a Surge in New Covid-19 Infections

TAIPEI, Taiwan — Closed schools and restaurants offering takeout only. Lines around the block at testing sites. Politicians on television urging the public to stay calm.

If the scenes around Taiwan this week have a distinctly early pandemic feel, it is because the coronavirus is only now washing up on the island’s shores in force. A crush of new infections has brought a swift end to the Covid-free normality that residents had been enjoying for more than a year.

By shutting its borders early and requiring two-week quarantines of nearly everyone who arrives from overseas, Taiwan had been managing to keep life on the island mostly unfettered. But all that changed after enough infections slipped past those high walls to cause community outbreaks.

For most of the past week, the government has ordered residents to stay home whenever possible and to wear masks outdoors, though it has not declared a total lockdown. Local authorities are ramping up rapid testing, though some health experts worry that too few tests are being done to stay ahead of the virus’s spread.

1,290 Covid-19 cases and 12 deaths during the entire pandemic.

Adding to the concern: Only around 1 percent of the island’s 23.5 million residents have been vaccinated against the virus so far.

happily shielded from its worst ravages.

Eight months passed last year without a single case of community transmission until an infection in December snapped the streak. Even after that, local infections cropped up only sporadically for months.

Then the tide shifted — gradually, then suddenly.

On April 14, the government began allowing crew members for Taiwanese airlines to quarantine at home for just three days after arriving on long-haul flights, down from the previous requirement of five days.

more pilots and their family members were testing positive, as were employees at a quarantine hotel.

On May 10, a pilot who had been in the United States tested positive after completing his three-day quarantine, but not before he had visited a pub and a restaurant in Taipei.

ordered into rolling 14-day home quarantines. But it was probably too late. A cluster of infections began to emerge among workers and patrons at so-called hostess bars in Taipei’s Wanhua District.

By the end of the week, daily case numbers had soared into the triple digits.

So far, the search for new infections has been concentrated in the populous cities of Taipei and New Taipei, where more than 1,600 people can receive rapid testing each day. Hospitals are also providing slower testing services.

Dr. Chiang Kuan-yu, 37, a physician at Taipei City Hospital, went to Wanhua District on Monday to help run a testing site there. He said there had been big crowds over the weekend, when the case numbers first started to rise. Some people had to wait an extra day to get tested.

“Now there are more resources for testing, so we can keep up better,” Dr. Chiang said.

Chen Shih-chung, Taiwan’s health minister and head of its Central Epidemic Command Center, has urged those with no Covid-19 symptoms and no history of contact to not even come to testing sites, lest they become infected there.

“This only will slow down our search for possible spreaders,” Mr. Chen said in a news briefing. “Don’t go there thinking, ‘Oh, maybe I’m infected, maybe it’s best that I get tested.’ You absolutely must not come.”

early March, and it has since been gradually immunizing health workers and other priority groups. Officials say doses of the Moderna vaccine will arrive soon. Several Taiwanese companies are also developing vaccines.

Taiwanese authorities began working with domestic vaccine producers in January 2020, after the coronavirus’s genetic sequence was made available and before the Chinese city of Wuhan went into lockdown.

“Taiwan got started extremely early,” said Dr. Ho Mei-shang, a research fellow at the Institute of Biomedical Sciences at Academia Sinica in Taipei who was involved with the government’s vaccine efforts. “We said at the time, ‘Whatever the vaccine ends up being, we want make it ourselves as quickly as possible.’”

But Taiwan’s insistence on developing and producing its own immunizations may have made officials less quick to snap up overseas vaccines when those started becoming available, Dr. Ho said.

“And then,” she said, “by the beginning of this year, when the pandemic was so severe in so many countries, we just said we’ll wait a little.”

Even after the AstraZeneca vaccine first became available in Taiwan, the low case count meant many people felt no urgent need to get immunized.

Still, Dr. Ho said she was heartened to see how quickly people in Taiwan were adjusting to the new restrictions on daily life, even after such a carefree past year.

Recently, she went for a run at 10 p.m. and forgot to wear her mask at first. But she noticed that even at that hour, everyone else who was out walking and exercising was masked up.

“This is a state of affairs,” she said, “that really sets Taiwan apart.”

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Biden Dips Into U.S. Vaccine Supply to Send 20 Million Doses Abroad

WASHINGTON — President Biden, heeding widespread calls to step up his response to the pandemic’s surge abroad, said on Monday that his administration would send 20 million doses of federally authorized coronavirus vaccine overseas in June — the first time he has pledged to give away doses that could be used in the United States.

The donation is another step toward what Mr. Biden promised would be an “entirely new effort” to increase vaccine supplies and vastly expand manufacturing capacity, most of it in the United States. He also put Jeffrey Zients, the White House coronavirus response coordinator, in charge of developing a global strategy.

“We know America will never be fully safe until the pandemic that’s raging globally is under control,” Mr. Biden said in a brief appearance at the White House. “No ocean’s wide enough, no wall’s high enough, to keep us safe.”

With new cases and deaths plummeting as vaccination rates rise in the United States, the epicenter of the crisis has moved to India and other nations. A growing and bipartisan chorus of diplomats, health experts and business leaders has been pushing the president to do more to end what the AIDS activist Asia Russell calls “vaccine apartheid.”

There is a huge disconnect growing where, in some countries with the highest vaccination rates, there appears to be a mind-set that the pandemic is over, while others are experiencing huge waves of infection,” Dr. Tedros said.

Variants like B.1.617, first discovered in India and recently designated a variant of concern by the W.H.O., are contributing to the spread of infections and worry many researchers.

Dr. Tedros called for well-supplied nations to send more of their vaccine allocations to harder-hit countries, and for vaccine developers and manufacturers to hasten delivery of hundreds of millions of doses to Covax, an international initiative dedicated to equitable distribution of the vaccine, noting an appeal by Henrietta Fore, UNICEF’s executive director.

Mr. Biden took office vowing to restore the United States as a leader in global public health, and he has taken certain steps to do so: rejoining the World Health Organization, pledging $4 billion to an international vaccine effort and providing financial support to help Biological E, a vaccine manufacturer in India, produce at least one billion doses of coronavirus vaccines by the end of 2022.

To broaden supply further, Mr. Biden recently announced he would support waiving intellectual property protections for coronavirus vaccines. But activists say simply supporting the waiver is not enough; Mr. Biden must create the conditions for pharmaceutical companies to transfer their intellectual property to vaccine makers overseas, they argue. They view his efforts as piecemeal.

“We’re after 100 days into the administration, and what Biden should be delivering is a global battle plan against vaccine apartheid, and the announcement today is lines on a Post-it note,” Ms. Russell said, adding, “There must be a global strategy led by the U.S. that’s based on technology transfer, on forcing pharma to come to the table to share the recipe.”

assert that a fix is already at hand as they aggressively expand production lines and contract with counterparts around the world to yield billions of additional doses.

An open letter to the president, made public last week by a bipartisan group including business leaders, diplomats and a former defense secretary, argued that such a waiver “would make little difference and could do harm.”

While global health activists are strongly in favor of the waiver, some said they welcomed the views of the business community. They see clear parallels to their work fighting the global AIDS epidemic.

“It shows an unprecedented willingness of pharma and its allies in the private sector to admit what all of us having been saying for months — the private sector alone cannot and will not ensure global vaccine access,” James Krellenstein, a founder of PrEP4All, a nonprofit aimed at ensuring universal access to H.I.V. prevention and treatment, wrote in an email on Sunday. “It really shifts the burden to the Biden administration,” he added.

The organizer of the open letter, Hank Greenberg, the chairman of Starr Companies and former chairman of American International Group, the insurance industry giant, said in an interview on Monday that Mr. Biden’s announcement did not go far enough.

Mr. Greenberg, 96, a veteran of World War II, said he was inspired to write after a former chief executive of an A.I.G. subsidiary who later became the ambassador from the Philippines to the United States told him he was not able to get vaccinated. Like Mr. Biden, he used language that evoked the war effort.

“If we don’t do it,” he asked, “who will?”

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Why Vaccinating the World Against Covid-19 Will Be Hard

In delivering vaccines, pharmaceutical companies aided by monumental government investments have given humanity a miraculous shot at liberation from the worst pandemic in a century.

But wealthy countries have captured an overwhelming share of the benefit. Only 0.3 percent of the vaccine doses administered globally have been given in the 29 poorest countries, home to about 9 percent of the world’s population.

Vaccine manufacturers assert that a fix is already at hand as they aggressively expand production lines and contract with counterparts around the world to yield billions of additional doses. Each month, 400 million to 500 million doses of the vaccines from Moderna, Pfizer and Johnson & Johnson are now being produced, according to an American official with knowledge of global supply.

But the world is nowhere close to having enough. About 11 billion shots are needed to vaccinate 70 percent of the world’s population, the rough threshold needed for herd immunity, researchers at Duke University estimate. Yet, so far, only a small fraction of that has been produced. While global production is difficult to measure, the analytics firm Airfinity estimates the total so far at 1.7 billion doses.

dangerous new variants emerge, requiring booster shots and reformulated vaccines, demand could dramatically increase, intensifying the imperative for every country to lock up supply for its own people.

The only way around the zero-sum competition for doses is to greatly expand the global supply of vaccines. On that point, nearly everyone agrees.

But what is the fastest way to make that happen? On that question, divisions remain stark, undermining collective efforts to end the pandemic.

Some health experts argue that the only way to avert catastrophe is to force drug giants to relax their grip on their secrets and enlist many more manufacturers in making vaccines. In place of the existing arrangement — in which drug companies set up partnerships on their terms, while setting the prices of their vaccines — world leaders could compel or persuade the industry to cooperate with more companies to yield additional doses at rates affordable to poor countries.

Those advocating such intervention have focused on two primary approaches: waiving patents to allow many more manufacturers to copy existing vaccines, and requiring the pharmaceutical companies to transfer their technology — that is, help other manufacturers learn to replicate their products.

more than 100 countries in asking the W.T.O. to partially set aside vaccine patents.

But the European Union has signaled its intent to oppose waivers and support only voluntary tech transfers, essentially taking the same position as the pharmaceutical industry, whose aggressive lobbying has heavily shaped the rules in its favor.

Some experts warn that revoking intellectual property rules could disrupt the industry, slowing its efforts to deliver vaccines — like reorganizing the fire department amid an inferno.

“We need them to scale up and deliver,” said Simon J. Evenett, an expert on trade and economic development at the University of St. Gallen in Switzerland. “We have this huge production ramp up. Nothing should get in the way to threaten it.”

Others counter that trusting the pharmaceutical industry to provide the world with vaccines helped create the current chasm between vaccine haves and have-nots.

The world should not put poorer countries “in this position of essentially having to go begging, or waiting for donations of small amounts of vaccine,” said Dr. Chris Beyrer, senior scientific liaison to the Covid-19 Prevention Network. “The model of charity is, I think, an unacceptable model.”

halting vaccine exports a month ago. Now, as a wave of death ravages the largely unvaccinated Indian population, the government is drawing fire at home for having let go of doses.

poses universal risks by allowing variants to take hold, forcing the world into an endless cycle of pharmaceutical catch-up.

“It needs to be global leaders functioning as a unit, to say that vaccine is a form of global security,” said Dr. Rebecca Weintraub, a global health expert at Harvard Medical School. She suggested that the G7, the group of leading economies, could lead such a campaign and finance it when the members convene in England next month.

Pfizer expects to sell $26 billion worth of Covid vaccines this year; Moderna forecasts that its sales of Covid vaccines will exceed $19 billion for 2021.

History also challenges industry claims that blanket global patent rights are a requirement for the creation of new medicines. Until the mid-1990s, drug makers could patent their products only in the wealthiest markets, while negotiating licenses that allowed companies in other parts of the world to make generic versions.

Even in that era, drug companies continued to innovate. And they continued to prosper even with the later waivers on H.I.V. drugs.

“At the time, it rattled a lot of people, like ‘How could you do that? It’s going to destroy the pharmaceutical industry,’” recalled Dr. Anthony S. Fauci, President Biden’s chief medical adviser for the pandemic. “It didn’t destroy them at all. They continue to make billions of dollars.”

Leaders in the wealthiest Western nations have endorsed more equitable distribution of vaccines for this latest scourge. But the imperative to ensure ample supplies for their own nations has won out as the virus killed hundreds of thousands of their own people, devastated economies, and sowed despair.

The drug companies have also promised more support for poorer nations. AstraZeneca’s vaccine has been the primary supply for Covax, and the company says it has sold its doses at a nonprofit price.

stumbled, falling short of production targets. And producing the new class of mRNA vaccines, like those from Pfizer-BioNTech and Moderna, is complicated.

Where pharmaceutical companies have struck deals with partners, the pace of production has frequently disappointed.

“Even with voluntary licensing and technology transfer, it’s not easy to make complex vaccines,” said Dr. Krishna Udayakumar, director of the Duke Global Health Innovation Center.

Much of the global capacity for vaccine manufacturing is already being used to produce other lifesaving inoculations, he added.

But other health experts accuse major pharmaceutical companies of exaggerating the manufacturing challenges to protect their monopoly power, and implying that developing countries lack the acumen to master sophisticated techniques is “an offensive and a racist notion,” said Matthew Kavanagh, director of the Global Health Policy and Politics Initiative at Georgetown University.

With no clear path forward, Ms. Okonjo-Iweala, the W.T.O. director-general, expressed hope that the Indian and South African patent-waiver proposal can be a starting point for dialogue.

“I believe we can come to a pragmatic outcome,” she said. “The disparity is just too much.”

Peter S. Goodman reported from London, Apoorva Mandavilli from New York, Rebecca Robbins from Bellingham, Wash., and Matina Stevis-Gridneff from Brussels. Noah Weiland contributed reporting from New York.

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What Would It Take to Vaccinate the World Against Covid?

In delivering vaccines, pharmaceutical companies aided by monumental government investments have given humanity a miraculous shot at liberation from the worst pandemic in a century.

But wealthy countries have captured an overwhelming share of the benefit. Only 0.3 percent of the vaccine doses administered globally have been given in the 29 poorest countries, home to about 9 percent of the world’s population.

Vaccine manufacturers assert that a fix is already at hand as they aggressively expand production lines and contract with counterparts around the world to yield billions of additional doses. Each month, 400 million to 500 million doses of the vaccines from Moderna, Pfizer and Johnson & Johnson are now being produced, according to an American official with knowledge of global supply.

But the world is nowhere close to having enough. About 11 billion shots are needed to vaccinate 70 percent of the world’s population, the rough threshold needed for herd immunity, researchers at Duke University estimate. Yet, so far, only a small fraction of that has been produced. While global production is difficult to measure, the analytics firm Airfinity estimates the total so far at 1.7 billion doses.

more than 100 countries in asking the W.T.O. to partially set aside vaccine patents.

But the European Union has signaled its intent to oppose waivers and support only voluntary tech transfers, essentially taking the same position as the pharmaceutical industry, whose aggressive lobbying has heavily shaped the rules in its favor.

Some experts warn that revoking intellectual property rules could disrupt the industry, slowing its efforts to deliver vaccines — like reorganizing the fire department amid an inferno.

“We need them to scale up and deliver,” said Simon J. Evenett, an expert on trade and economic development at the University of St. Gallen in Switzerland. “We have this huge production ramp up. Nothing should get in the way to threaten it.”

Others counter that trusting the pharmaceutical industry to provide the world with vaccines helped create the current chasm between vaccine haves and have-nots.

The world should not put poorer countries “in this position of essentially having to go begging, or waiting for donations of small amounts of vaccine,” said Dr. Chris Beyrer, senior scientific liaison to the Covid-19 Prevention Network. “The model of charity is, I think, an unacceptable model.”

Pfizer expects to sell $26 billion worth of Covid vaccines this year; Moderna forecasts that its sales of Covid vaccines will exceed $19 billion for 2021.

History also challenges industry claims that blanket global patent rights are a requirement for the creation of new medicines. Until the mid-1990s, drug makers could patent their products only in the wealthiest markets, while negotiating licenses that allowed companies in other parts of the world to make generic versions.

Even in that era, drug companies continued to innovate. And they continued to prosper even with the later waivers on H.I.V. drugs.

“At the time, it rattled a lot of people, like ‘How could you do that? It’s going to destroy the pharmaceutical industry,’” recalled Dr. Anthony S. Fauci, President Biden’s chief medical adviser for the pandemic. “It didn’t destroy them at all. They continue to make billions of dollars.”

Leaders in the wealthiest Western nations have endorsed more equitable distribution of vaccines for this latest scourge. But the imperative to ensure ample supplies for their own nations has won out as the virus killed hundreds of thousands of their own people, devastated economies, and sowed despair.

The drug companies have also promised more support for poorer nations. AstraZeneca’s vaccine has been the primary supply for Covax, and the company says it has sold its doses at a nonprofit price.

stumbled, falling short of production targets. And producing the new class of mRNA vaccines, like those from Pfizer-BioNTech and Moderna, is complicated.

Where pharmaceutical companies have struck deals with partners, the pace of production has frequently disappointed.

“Even with voluntary licensing and technology transfer, it’s not easy to make complex vaccines,” said Dr. Krishna Udayakumar, director of the Duke Global Health Innovation Center.

Much of the global capacity for vaccine manufacturing is already being used to produce other lifesaving inoculations, he added.

But other health experts accuse major pharmaceutical companies of exaggerating the manufacturing challenges to protect their monopoly power, and implying that developing countries lack the acumen to master sophisticated techniques is “an offensive and a racist notion,” said Matthew Kavanagh, director of the Global Health Policy and Politics Initiative at Georgetown University.

With no clear path forward, Ms. Okonjo-Iweala, the W.T.O. director-general, expressed hope that the Indian and South African patent-waiver proposal can be a starting point for dialogue.

“I believe we can come to a pragmatic outcome,” she said. “The disparity is just too much.”

Peter S. Goodman reported from London, Apoorva Mandavilli from New York, Rebecca Robbins from Bellingham, Wash., and Matina Stevis-Gridneff from Brussels. Noah Weiland contributed reporting from New York.

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Why the C.D.C. Changed Its Advice on Masks

Advice from federal health officials that fully vaccinated people could drop their masks in most settings came as a surprise to Americans, from state officials to scientific experts. Even the White House got less than a day’s notice from the Centers for Disease Control and Prevention, the press secretary, Jen Psaki, said at a news briefing on Friday.

“The C.D.C., the doctors and medical experts there, are the ones who determined what this guidance would be based on their own data, and what the timeline would be,” Ms. Psaki said. “That was not a decision directed by or made by the White House.”

For months, federal officials have vigorously warned that wearing masks and social distancing were necessary to contain the pandemic. So what changed?

Introducing the new recommendations on Thursday, Dr. Rochelle P. Walensky, the C.D.C. director, cited two recent scientific findings as significant factors: Few vaccinated people become infected with the virus, and transmission seems rarer still; and the vaccines appear to be effective against all known variants of the coronavirus.

There is no doubt at this point that the vaccines are powerful. On Friday, the C.D.C. released results from another large study showing that the vaccines made by Pfizer-BioNTech and Moderna are 94 percent effective in preventing symptomatic illness in those who were fully vaccinated, and 82 percent effective even in those only partly vaccinated.

“The science is quite clear on this,” said Zoë McLaren, a health policy expert at the University of Maryland, Baltimore County. Mounting evidence indicates that people who are vaccinated are highly unlikely to catch or transmit the virus, she noted.

The risk “is definitely not zero, but it’s clear that it’s very low,” she said.

One of the lingering concerns among scientists had been that even a vaccinated person might carry the virus — perhaps briefly, without symptoms — and spread it to others. But C.D.C. research, including the new study, has consistently found few infections among those who received the Pfizer-BioNTech and Moderna vaccines.

“This study, added to the many studies that preceded it, was pivotal to C.D.C. changing its recommendations for those who are fully vaccinated against Covid-19,” Dr. Walensky said in a statement on Friday.

Other recent studies confirm that people who are infected after vaccination carry too little virus to infect others, said Florian Krammer, a virologist at the Icahn School of Medicine at Mount Sinai.

“It’s really hard to even sequence the virus sometimes because there’s very little virus, and it’s there for a short period of time,” he said.

Still, most of the data has been gathered on the Pfizer-BioNTech and Moderna vaccines, Dr. Krammer cautioned. Because Johnson & Johnson’s vaccine was authorized later, there are fewer studies assessing its effectiveness.

In clinical trials, the Johnson & Johnson vaccine had 72 percent efficacy — lower than the figure for the Pfizer and Moderna vaccines. And effectiveness was measured in terms of moderate and severe disease, rather than mild disease.

“It’s a very good vaccine, and I’m sure it will save many, many, many lives,” Dr. Krammer said. “But we need more data on how well the J.&J. vaccine prevents infection, and how well it prevents transmission.”

Variants of the virus have been a particular worry for scientists. While Dr. Walensky cited evidence showing that the mRNA vaccines like those from Pfizer and Moderna are effective against the variants circulating in the United States, there is little data about variants and the Johnson & Johnson vaccine. And new variants are emerging constantly.

“I’m not at all saying that this is now a big problem,” Dr. Krammer said. But before lifting the masking requirements, “I might have waited a little bit longer to look at the numbers.”

In a statement on Friday, a C.D.C. spokesman said, “All of the authorized vaccines provide strong protection against serious illness, hospitalization, and death, and we are accumulating data that our authorized vaccines are effective against the variants that are circulating in this country.”

Fully immunized people are unlikely to get seriously ill, even if they are infected with the coronavirus. The risk of infection is greater for the people around them — unvaccinated children and adults, or vaccinated people who remain unprotected because of a medical condition or treatment.

C.D.C. officials said they weighed those factors and were confident in their assessment of the science. And the new advice has other salutary effects, rewarding fully immunized people by giving them permission to end their social isolation — and perhaps incentivizing others to opt for vaccination.

The new advice “signals that we really are on the final stretch here, and I think that’s a very good thing for people,” said Dr. Joshua Sharfstein, the vice dean for public health practice and community engagement at Johns Hopkins University Bloomberg School of Health.

“It’s unlikely that we’re going to have another huge surge in cases,” he added. “But will the final stretch last for weeks or months is still a question.”

The difficulty with the new recommendations, he and other experts said, is not so much the science underpinning them as their implementation.

Leaders at the state, city and county levels still have the authority to require masks even for vaccinated people, as the C.D.C. was quick to acknowledge on Thursday. After the agency’s announcement, some states instantly lifted mask mandates, while others said they would need more time to weigh the evidence.

But in states without mask mandates, the onus of checking vaccination status will fall on shopkeepers, restaurant workers, school officials and workplace managers.

“Without a means to verify vaccination, we will have to rely on an honor system,” said Caitlin Rivers, an epidemiologist at Johns Hopkins University.

The number of cases in the country is the lowest it has been since September, and many experts support lifting mask mandates in much of the country. But doing so will be riskier in places like Michigan, where there are more cases, and for people who are unprotected, including children under 12 and people with a weak immune systems, Dr. Rivers said.

“People who are unvaccinated should continue to wear masks in public indoors and avoid crowds,” she said.

In Nacogdoches, Texas, Dr. Ahammed Hashim fretted that only 36 percent of the population was immunized and the pace seemed to have stalled. And yet only one or two people in 10 in the local shops wore masks.

“I think the C.D.C. might send a wrong message saying that everything’s OK,” said Dr. Hashim, a pulmonologist. “It would feel much better if we had a 60 or 70 percent vaccination.”

The C.D.C.’s guidance is intended for fully vaccinated individuals, and should only be interpreted as such, Dr. Sharfstein cautioned. Nationwide, only 36 percent of the population is fully vaccinated.

“What we’re just seeing is a little bit of the distance between advice that is entirely appropriate for people who are vaccinated, and the reality that there are places that still are seeing viral transmission and a lot of people who aren’t vaccinated,” he said.

Individuals may make choices based on their perception of their own risks, but state and local leaders must decide what’s best for the community based on the rate of infections. “Those are two different things,” Dr. Sharfstein said. “And when they get conflated, that’s when people may make bad judgments about policy.”

The new guidelines should serve as a reminder to health officials to step up their outreach and investment to ensure that everyone has access to vaccines, Dr. McLaren said. Parents of children under 12 should continue to urge them to wear masks indoors.

The C.D.C.’s new policy shifts the onus onto the immunocompromised as well, to protect themselves from unmasked and unvaccinated people.

“When we make policy, we need to balance the needs and desires of everyone,” Dr. McLaren said. “We could keep masking forever, but there are benefits to getting back to a life that looks more normal.”

Health officials should emphasize that the situation may yet change, and official recommendations with it, she added: “We really need to practice being good at responding to changing situations.”

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Covid Vaccines Protect Pregnant Women, Study Confirms

The vaccines produced similar responses in all three groups of women, eliciting both antibody and T-cell responses against the coronavirus, the scientists found. Of particular note, experts said, was the fact that the shots produced high levels of neutralizing antibodies, which can prevent the virus from entering cells, in both pregnant and nonpregnant women.

“Clearly, the vaccines were working in these people,” said Akiko Iwasaki, an immunologist at Yale University who was not involved in the research. “These levels are expected to be quite protective.”

The researchers also found neutralizing antibodies in the breast milk of vaccinated mothers and in umbilical cord blood collected from infants at delivery. “Vaccination of pregnant people and lactating people actually leads to transfer of some immunity to their newborns and lactating infants,” said Dr. Ai-ris Y. Collier, a physician-scientist at Beth Israel who is the first author of the paper.

The results are “really encouraging,” Dr. Iwasaki said. “There is this added benefit of conferring protective antibodies to the newborn and the fetus, which is all the more reason to get vaccinated.”

The scientists also measured the women’s immune responses to two variants of concern: B.1.1.7, which was first identified in Britain, and B.1.351, which was first identified in South Africa. All three groups of women produced antibody and T-cell responses to both variants after vaccination, although their antibody responses were weaker against the variants, especially B.1.351, than against the original strain of the virus, according to the study.

“These women developed immune responses to the variants, although the asterisk is that the antibody responses were reduced several-fold,” said Dr. Dan Barouch, a study author and virologist at Beth Israel. (Dr. Barouch and his colleagues developed the Johnson & Johnson vaccine, which was not included in this study.)

“Overall, it’s good news,” he added. “And it increases the data that suggests that there is a substantial benefit for pregnant women to be vaccinated.”

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