seasonal dust storms that sweep into the country from Mongolia and northern China.

“Generally speaking, Koreans until recently believed that mask wearing was a sort of ‘Japanese practice,’ not ours,” he said.

In Hong Kong, where 299 people died during the SARS epidemic of 2002-3, the experience of universal masking against that coronavirus helped create a “cultural familiarity” with a practice that was also common during episodes of severe air pollution, Mr. De Kai said.

“It was a big reminder to people that masks are important not only to protect yourself from the pollution but also to avoid infecting those around you,” he said.

In Taiwan, SARS and recent air pollution were the two main factors that prompted people there to develop the habit of mask wearing, said Yeh Ming-Jui, a professor of public health at National Taiwan University in Taipei.

Professor Yeh said he believed mask wearing was not more widespread in the West because people there had no immediate memories of a severe pandemic — at least until now.

“The experience and health practices of past generations have been gradually forgotten,” he said.

Amy Chang Chien contributed reporting from Taipei.

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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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Virus Variants Can Infect Mice, Scientists Report

Bats, humans, monkeys, minks, big cats and big apes — the coronavirus can make a home in many different animals. But now the list of potential hosts has expanded to include mice, according to an unnerving new study.

Infected rodents pose no immediate risk to people, even in cities like London and New York, where they are ubiquitous and unwelcome occupants of subway stations, basements and backyards.

Still, the finding is worrying. Along with previous work, it suggests that new mutations are giving the virus the ability to replicate in a wider array of animal species, experts said.

“The virus is changing, and unfortunately it’s changing pretty fast,” said Timothy Sheahan, a virologist at the University of North Carolina at Chapel Hill, who was not involved in the new study.

only animals known to be able to catch the coronavirus from humans and pass it back. In early November, Denmark culled 17 million farmed mink to prevent the virus from evolving into dangerous new variants in the animals.

More recently, researchers found that B.1.1.7 infections in domesticated cats and dogs can cause the pets to develop heart problems similar to those seen in people with Covid-19.

To establish a successful infection, the coronavirus must bind to a protein on the surface of animal cells, gain entry into the cells, and exploit their machinery to make copies of itself. The virus must also evade the immune system’s early attempts at thwarting the infection.

Given all those requirements, it is “quite extraordinary” that the coronavirus can infect so many species, said Vincent Munster, a virologist at the National Institute of Allergy and Infectious Diseases. “Typically, viruses have a more curtailed host range.”

Mice are a known reservoir for hantavirus, which causes a rare and deadly disease in people. Even though the coronavirus variants don’t seem to be able to jump from mice to people, there is potential for them to spread among rodents, evolve into new variants, and then infect people again, Dr. Munster said.

black-footed ferrets. “This virus seems to be able to surprise us more than anything else, or any other previous virus,” Dr. Munster said. “We have to err on the side of caution.”

Dr. Sheahan said he was more concerned about transmission to people from farm animals and pets than from mice.

“You’re not catching wild mice in your house and snuggling — getting all up in their face and sharing the same airspace, like maybe with your cat or your dog,” he said. “I’d be more worried about wild or domestic animals with which we have a more intimate relationship.”

But he and other experts said the results emphasized the need to closely monitor the rapid changes in the virus.

“It’s like a moving target — it’s crazy,” he added. “There’s nothing we can do about it, other than try and get people vaccinated really fast.”

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Getting One Vaccine Is Good. How About Mix-and-Match?

In January, Britain made a change to its vaccine guidelines that shocked many health experts: If the second dose of one vaccine wasn’t available, patients could be given a different one.

The new rule was based on sheer guesswork; there was no scientific data at the time demonstrating that mixing two coronavirus vaccines was safe and effective. But that may change soon.

In February, researchers at the University of Oxford began a trial in which volunteers received a dose of the Pfizer-BioNTech vaccine followed by a dose of AstraZeneca’s formulation, or vice versa. This month, the researchers will start analyzing the blood of the subjects to see how well the mix-and-match approach works.

As growing numbers of vaccines are being authorized, researchers are testing other combinations. A few are in clinical trials, while others are being tested in animals for now.

created an Ebola vaccine whose first dose contained a virus called an adenovirus. The second shot used another virus, called vesicular stomatitis virus.

When the Covid-19 pandemic began last year, the Gamaleya researchers used a similar strategy to create vaccines against the new coronavirus. The first dose used the same adenovirus as in their Ebola vaccine, called Ad5. The second dose contained a different human adenovirus, Ad26. The researches inserted a gene into both viruses for the protein on the surface of the coronavirus, called spike.

Studies revealed that the vaccine, now known as Sputnik V, provided a strong defense against Covid-19. In clinical trials, the researchers found that it had an efficacy of 91.6 percent. Sputnik V is now in use in Russia and 56 other countries.

Recently, the Gamaleya institute joined forces with AstraZeneca, which makes its own Covid-19 vaccine. AstraZeneca’s consists of two doses of a chimpanzee adenovirus called ChAdOx1. Last week, the company reported that its vaccine had an efficacy of 76 percent.

found that the mixture worked better than two doses of the spike or of the R.B.D.

The researchers suspect that the first dose produces a broad range of antibodies that can stick to spots along the length of the spike protein, and that the second dose delivers a big supply of particularly potent antibodies to the tip of the spike. Together, the assortment of antibodies does a better job of stopping the coronavirus.

held back exports of vaccines to other countries as it grappled with a surge of Covid-19. For countries that were counting on those vaccines, a safe alternative for second doses could save lives.

After Britain was criticized in January for suggesting that vaccines could be mixed, researchers at the University of Oxford set out to put the idea to a formal test. In a trial called Com-Cov, they recruited 830 volunteers to test the two vaccines authorized by the British government: AstraZeneca’s adenovirus-based vaccine and the vaccine by Pfizer-BioNTech.

Pfizer-BioNTech’s vaccine uses a fundamentally different technology to produce spike proteins in the body. It contains tiny bubbles with genetic molecules called RNA. Once the bubbles fuse to cells, the cells use the RNA to make spike proteins.

stronger immune responses than mice that received the same vaccine for both doses.

Whether scientists carry out more experiments on other vaccines will depend on the willingness of the vaccine manufacturers. “You’re requiring quite large pharmaceutical companies to play nice together,” Dr. Wheatley said.

Dr. Bernard Moss, a virologist at the National Institute of Allergy and Infectious Diseases, suspects that a number of companies will be willing to let their vaccines be tested in combinations. “It’s always better to be a part of something that is going to be used,” he said, “than to wholly own something that isn’t.”

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AstraZeneca Releases Fuller Data Backing Its Vaccine

AstraZeneca reiterated on Wednesday that its Covid-19 vaccine was very effective at preventing the disease, based on more recent data than was included when the company announced the interim results of its U.S. clinical trial on Monday.

The company said in a news release that its vaccine was 76 percent effective at preventing Covid-19. That is slightly lower than the number that the company announced earlier this week.

The new results strengthen the scientific case for the embattled vaccine. But they may not repair the damage to AstraZeneca’s credibility after U.S. health officials and independent monitors issued an extraordinary rebuke of the company for not counting some Covid-19 cases when it announced its initial findings this week.

In a news release on Wednesday, the company said complete results from its 32,000-person study showed that its vaccine was 76 percent effective. On Monday, the company had said the vaccine appeared to be 79 percent effective, based on an interim look at 141 Covid-19 cases that had turned up among volunteers before Feb. 17. The latest finding was based on 190 trial participants who had gotten sick with Covid-19.

public statement.

said last week that a review had found the shot to be safe after a small number of people who had recently been inoculated developed blood clots and abnormal bleeding. The U.S. trial did not turn up any signs of such safety problems.

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U.S. health officials question results from AstraZeneca’s vaccine trial, less than a day after they’re released.

Federal health officials said early Tuesday that results from a U.S. trial of AstraZeneca’s Covid-19 vaccine may have relied on “outdated information” that “may have provided an incomplete view of the efficacy data,” casting doubt on an announcement on Monday that had been seen as good news for the British-Swedish company as well as the global vaccination drive.

In a highly unusual statement released after midnight, the National Institute of Allergy and Infectious Diseases said that the data and safety monitoring board, an independent panel of medical experts under the National Institutes of Health that has been helping to oversee AstraZeneca’s U.S. trial, had notified government agencies and AstraZeneca late Monday that it was “concerned” by information the company had released that morning.

The institute urged AstraZeneca to work with the monitoring board “to review the efficacy data and ensure the most accurate, up-to-date efficacy data be made public as quickly as possible.”

AstraZeneca did not immediately return a request for comment early Tuesday.

In a news release on Monday announcing the results of the U.S. trial, the company said that the vaccine it developed with the University of Oxford was 79 percent effective against Covid-19, a higher figure than observed in previous trials, and completely prevented the worst outcomes from the disease. The long-anticipated results were seen as encouraging global confidence in the vaccine, which was shaken this month when more than a dozen countries, mostly in Europe, temporarily suspended the shot’s use over concerns about possible rare side effects. After conducting a review, Europe’s top drug regulator said last week that the shot was “safe and effective.”

more than 17 million in Britain and the European Union, almost all without serious side effects. In an effort to increase public confidence, many European political leaders have gotten the injections in recent days. The AstraZeneca vaccine has also been administered in the past week to leaders in South Korea, Taiwan and Thailand.

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U.S. Health Officials Question AstraZeneca Vaccine Trial Results

Federal health officials said early Tuesday that results from a U.S. trial of AstraZeneca’s Covid-19 vaccine may have relied on “outdated information” that “may have provided an incomplete view of the efficacy data,” casting doubt on an announcement on Monday that had been seen as good news for the British-Swedish company as well as the global vaccination drive.

In a highly unusual statement released after midnight, the National Institute of Allergy and Infectious Diseases said that the data and safety monitoring board, an independent panel of medical experts under the National Institutes of Health that has been helping to oversee AstraZeneca’s U.S. trial, had notified government agencies and AstraZeneca late Monday that it was “concerned” by information the company had released that morning.

The institute urged AstraZeneca to work with the monitoring board “to review the efficacy data and ensure the most accurate, up-to-date efficacy data be made public as quickly as possible.”

AstraZeneca did not immediately return a request for comment early Tuesday.

In a news release on Monday announcing the results of the U.S. trial, the company said that the vaccine it developed with the University of Oxford was 79 percent effective against Covid-19, higher than observed in previous trials, and completely prevented the worst outcomes from the disease. The long-anticipated results were seen as encouraging global confidence in the vaccine, which was shaken this month when more than a dozen countries, mostly in Europe, temporarily suspended the shot’s use over concerns about possible rare side effects.

the design of its clinical trials, its results and safety issues. That skepticism carried over to last week, when senior officials at a number of federal health agencies grew suspicious about why AstraZeneca had not announced data from its U.S. study.

That U.S. trial, which involved more than 32,000 participants, was the largest test of its kind for the shot. The results AstraZeneca released on Monday were from an interim look at the data after 141 Covid-19 cases had turned up among volunteers.

The company did not disclose how up-to-date the data are. If the analysis was conducted on data from a month or two ago, it is possible that a more current look would present a different picture of the vaccine’s effectiveness and safety. The company has said it will provide the Food and Drug Administration with a more comprehensive, recent set of data than what it disclosed on Monday. Although no clinical trial is large enough to rule out extremely rare side effects, AstraZeneca reported that its study turned up no serious safety issues.

The fresh data may have arrived too late to make much difference in the United States, where the vaccine is not yet authorized and is unlikely to become available before May. By then, federal officials predict, there will be enough vaccine doses for all of the nation’s adults from the three vaccines that have already been authorized: Pfizer-BioNTech, Moderna and Johnson & Johnson.

Even so, the better-than-expected results were seen as a heartening turn for AstraZeneca’s shot, whose low cost and simple storage requirements have made it a vital piece of the drive to vaccinate the world.

said last week that the shot was “safe and effective,” having conducted a review after a small number of people who had recently been inoculated developed blood clots and abnormal bleeding. The U.S. trial did not turn up any sign of such problems, although some safety issues can only be detected in the real world, once a drug or vaccine has been widely used.

Many millions of people have received the AstraZeneca shot worldwide, including more than 17 million in Britain and the European Union, almost all without serious side effects. In an effort to increase public confidence, many European political leaders have gotten the injections in recent days. The AstraZeneca vaccine has also been administered in the past week to leaders in South Korea, Taiwan and Thailand.

AstraZeneca said on Monday that it would continue to analyze the new data and prepare to apply in the coming weeks for emergency authorization in the United States. The vaccine has already been approved in more than 70 countries, but clearance from American regulators would bolster its global reputation.

The statement from the infectious disease institute comes after a series of miscues and communication blunders by AstraZeneca dating to last year that have eroded American officials’ trust in the company.

Last summer, at least some top F.D.A. officials learned only from news reports that AstraZeneca had paused its Phase ⅔ vaccine trial in Britain after a participant developed neurological symptoms. Then in September, after another participant in the British study fell ill with similar symptoms, AstraZeneca halted its trials globally but failed to promptly notify the U.S. authorities.

The U.S. study was ultimately paused for seven weeks last fall, in part because AstraZeneca was slow to provide the F.D.A. with evidence that the vaccine had not caused the neurological symptoms. Investigators ultimately concluded that the illnesses could not be linked to the vaccine. Still, the delay was a key reason that AstraZeneca fell so far behind the three other manufacturers whose vaccines have been granted emergency authorization in the United States.

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