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Dr. Rochelle Walensky, the C.D.C. director, and Dr. Anthony Fauci, the nation’s leading infectious disease expert, testified on Capitol Hill before a House Select Committee overseeing the government’s coronavirus response.Amr Alfiky/The New York Times

Three top federal health officials appeared on Capitol Hill on Thursday and implored Americans to get vaccinated against the coronavirus, but said little about the investigation into whether the Johnson & Johnson vaccine may be linked to a small number of cases of rare blood clots, or when that vaccine might be put back into use.

“Hopefully we’ll get a decision quite soon, as to whether or not we can get back on track with this very effective vaccine,” Dr. Anthony S. Fauci, President Biden’s top medical adviser for the coronavirus, told a House panel.

Dr. Fauci’s comments came as the future of the Johnson & Johnson one-shot vaccine hung in the balance. Earlier this week, the Food and Drug Administration and the Centers for Disease Control and Prevention called for a pause in the use of the vaccine in the wake of reports of a small number of rare blood clots in recipients. Though it is unclear whether the vaccine was responsible for the clots, injections came to a sudden halt across the country.

On Wednesday, a C.D.C. advisory panel suggested that it would be a week to 10 days before they had enough information to assess the vaccine’s risks and could make a decision about its future.

The C.D.C. announced Thursday that the panel would hold another emergency meeting on April 23.

In the meantime, the officials — Dr. Fauci; Dr. Rochelle Walensky, the C.D.C. director; and Dr. David Kessler, who runs the Biden administration’s vaccine effort — urged Americans to continue to get vaccinated.

“I hope we can all come together and send that message,” especially amid the spread of worrisome variants, Dr. Kessler said, adding that the three federally authorized vaccines have “an excellent safety profile.”

During a hearing that lasted more than two hours, just one lawmaker — Representative Mark Green, Republican of Tennessee, who is a doctor — asked about the Johnson & Johnson vaccine. He urged the doctors to be careful when they talk about the investigation, saying he worried it would stoke fears that would discourage people from getting vaccinated.

Whatever science was discussed was overshadowed by partisan posturing and bickering. Republicans, fresh off a trip to the nation’s southern border, used the session to attack the Biden administration’s handling of the immigration crisis; they waved photographs of migrants living in crowded conditions, while complaining about testing rules for those entering the country.

Democrats blasted the Trump administration and asked softball questions.

“Dr. Fauci, I love you,” Representative Maxine Waters, Democrat of California, whose sister died of Covid-19, said at one point, after telling Dr. Fauci how much she relied on his counsel.

Dr. Fauci felt little love from Republicans, however. He and Representative Jim Jordan, Republican of Ohio, reprised their heated clash from the last time Dr. Fauci testified before the panel in July 2020.

“You’re ranting again,” Dr. Fauci said at one point.

“I’m not ranting,” Mr. Jordan replied.

“Yes, you are,” Dr. Fauci insisted.

With worrisome new variants of the virus spreading, Dr. Fauci and the others repeatedly described all three of the federally authorized vaccines — from Johnson & Johnson, Pfizer-BioNTech and Moderna — as highly safe and effective.

The reports of blood clots were the second recent blow to the Johnson & Johnson vaccine. Earlier this month, an ingredient mix-up at a Baltimore manufacturing plant owned by Emergent BioSolutions ruined up to 15 million doses of the vaccine. The F.D.A. is now inspecting the plant to see whether any vaccine doses manufactured there can be released to the public.

About 7.7 million Americans had received the Johnson & Johnson vaccine, accounting for less than 4 percent of the more than 198 million doses administered across the country. The Pfizer and Moderna vaccines are in much greater supply.

Officials note that the blood clots are extremely rare; the handful of cases represent less than one in one million recipients, although that incidence estimate could go up if more cases are reported.

Biden administration officials say that the absence of the Johnson & Johnson vaccine might not have a major impact on the U.S. vaccination campaign. But if use of the vaccine is severely restricted worldwide, it could prove disastrous for the global vaccination effort.

Health officials had hoped that the Johnson & Johnson vaccine, along with a similar vaccine developed by AstraZeneca, would help supply the world because they are less expensive and easier to store and handle than the Pfizer and Moderna vaccines.

But there have also been reports of rare blood clots in recipients of the AstraZeneca vaccine, leading a number of nations to reconsider its use. On Wednesday, Denmark, where two recipients suffered severe blood clots, permanently suspended use of the AstraZeneca vaccine.

Vaccinating in Los Angeles on Tuesday. Some doctors fear the Johnson & Johnson pause will increase hesitancy.
Allison Zaucha for The New York Times

After a day that many hoped would add clarity to the rollout of Johnson & Johnson’s troubled Covid-19 vaccine, the picture on Thursday is as muddy as ever in the United States.

The “pause” that U.S. health officials put in place on the use of the vaccine might now stay in place for seven to 10 days. It’s a decision with potentially painful consequences that could ripple worldwide.

After considering whether to reinstate the vaccine, a panel of expert advisers to the Centers for Disease Control and Prevention determined on Wednesday that it needed more time to assess a possible link to a rare but serious blood-clotting disorder.

In South Africa, health officials have stopped giving the Johnson & Johnson shot, two months after dropping another vaccine, from AstraZeneca. The European Union said it would not make any more purchases of the Johnson & Johnson vaccine or of AstraZeneca’s, which has raised similar concerns. If the perception takes hold that rich countries are dumping second-rate shots on poorer nations, suspicions about the efficacy of the vaccines could harden, slowing the worldwide rollout of desperately needed doses.

Already, doctors say, the recent pauses have heartened vaccine skeptics and made many others feel duped.

“People, especially those who were vaccinated, felt like they had been tricked in a way — they were asking, ‘How do we get rid of the vaccine in our body?’” said Precious Makiyi, a doctor and behavioral scientist in the Eastern African nation of Malawi, where health workers have been racing to empty their shelves of nearly expired AstraZeneca doses. “We fought so hard with vaccine messaging, but what has happened this past week has brought us back to square zero.”

In developed countries, too, the Johnson & Johnson woes could erode public confidence. The vaccine is considered ideal for rural and underserved communities because it requires only one shot and is easier to store.

“There was enthusiasm about it because it was a one-time thing,” said Jill Ramirez, chief executive of the Latino HealthCare Forum in Austin, Texas. “It was a really good opportunity for people to get the vaccine. But I feel uncomfortable signing people up for it.”

However, “Anytime there’s a disruption,” Ms. Ramirez said, “that throws a wrench into that trust. There’s going to be repercussions.”

As the Biden administration grapples with the fallout over the vaccine, three of its top health officials — Dr. Anthony S. Fauci, Dr. Rochelle Walensky and Dr. David Kessler — testified on Capitol Hill on Thursday morning before a House panel overseeing the government’s coronavirus response.

In their appearance, the doctors implored Americans to get vaccinated against the coronavirus, but said little about the Johnson & Johnson vaccine pause, or when that vaccine might be put back into use.

The C.D.C. advisory group’s emergency meeting on Wednesday was called to review the reports that had led to the pause on the Johnson & Johnson vaccine: Six cases of rare and severe blood clots in the brain in women ages 18 to 48, one of whom died.

All of the women had received the Johnson & Johnson vaccine before developing the clots, although it is unclear whether the vaccine was responsible. More than seven million people have had this shot in the United States. Public health officials and experts have emphasized that for most people, the benefits of the Covid vaccines far outweigh the risks, and that suspending use of some of them may do more harm than good.

The panel on Wednesday also learned of a seventh woman and a man who experienced the rare condition after receiving the Johnson & Johnson vaccine during clinical trials.

Advisory meetings usually end with a vote on whether or how to use a vaccine. But in this case, the members declined to vote after reviewing several options, including whether to limit use of the vaccine to older adults, saying that they did not have enough information to assess the potential risks.

Addressing the panel, Dr. Camille Kotton of Harvard Medical School warned that losing the Johnson & Johnson vaccine even temporarily represented a big blow to efforts to stop the pandemic, especially in underserved communities.

“Putting this vaccine on pause, for those of us that are frontline health care workers, has really been devastating,” she said.

AstraZeneca vaccine doses were received by airport workers in Kigali, Rwanda, as part of the Covax initiative, in March.
Muhizi Olivier/Associated Press

An effort to promote Covid-19 vaccination in the world’s lowest-income countries is trying to raise an additional $2 billion to help secure access to doses before they are snapped up by wealthier nations.

The initiative, known as Covax, plans to use the funds to reserve future production capacity to supply nations that have had a hard time getting enough doses, or in some cases, getting any at all.

“If you wait for the vaccine to be on the shelf, it will be too late,” said Marie-Ange Saraka-Yao, a managing director of GAVI, the Vaccine Alliance. “It will already be bought by someone else.”

GAVI and the United States hosted a fundraising event on Thursday for Covax and for a financing mechanism that promotes vaccinations for low-income countries; the event raised more than $380 million in pledges. Most of that was pledged by Sweden, with smaller amounts coming from other countries and private foundations.

One foundation, sponsored by Google, committed to giving $2.5 million in cash and $15 million in advertising credits, and also pledged technical support from the company’s engineers. “We hope others will step up and do more,” said Dr. Karen DeSalvo, chief health officer at Google Health.

The Biden administration has already said the United States would donate $2 billion to Covax in 2021 and another $2 billion next year.

Covax is coordinated by GAVI, the World Health Organization and the Coalition for Epidemic Preparedness Innovations. So far it has announced shipments of more than 38 million doses of coronavirus vaccines to 113 countries around the world.

Along with asking for more money, leaders of the effort have appealed to wealthier countries to donate a portion of their own vaccine supplies, to help rectify a vast and growing inequity in global distribution between richer and poorer parts of the world. New Zealand announced it would give doses for 800,000 people.

More than 841 million vaccine doses have been administered worldwide, equal to 11 doses for every 100 people. There is already a stark gap between vaccination programs in different countries, with many yet to report a single dose. And signs have begun to emerge that recent safety concerns about the AstraZeneca and Johnson & Johnson vaccines are undercutting vaccination campaigns across the world.

A firefighter administering a vaccination in Union City, Calif., on Thursday.
Jim Wilson/The New York Times

A third dose of the Covid-19 vaccine will “likely” be needed within a year of vaccination, followed by annual vaccinations, Pfizer’s chief executive said Thursday.

“There are vaccines like polio where one dose is enough, and there are vaccines like flu that you need every year,” Albert Bourla, the Pfizer chief, said during a pre-recorded conversation hosted by CVS Health Corp. which was posted Thursday. “The Covid virus looks more like the influenza virus than the polio virus.”

Mr. Bourla said that more data and sequencing would be needed to determine a re-vaccination protocol but that “a likely scenario” is “a third dose somewhere between six and 12 months, and from there it would be an annual re-vaccination.”

Coronavirus variants, like the one that first originated in Britain and is now the most common source of infection in the United States, will “play a key role” in how vaccines need to evolve, he said.

“It is extremely important to suppress the pool of people that can be susceptible to the virus,” Mr. Bourla said.

Dr. David Kessler, who runs the Biden administration’s vaccine effort, told a House committee on Thursday that the government was “planning for potential doses of vaccines, if they are needed.”

In February, Pfizer and its partner BioNTech said they planned to test a third shot as well as update their original vaccine. The Food and Drug Administration has said that vaccine developers will not need to conduct lengthy trials for vaccines that have been adapted to protect against variants.

On Tuesday, Moderna announced that its vaccine continues to provide strong protection in the United States against Covid-19 six months after it is given.

The company’s chief executive, Stéphane Bancel, told CNBC that he hoped to have booster shots ready by the fall “so that we protect people as we go into the next fall and winter season in the U.S.”

“We believe that we’re all going to need boosting,” he said.

Alex Gorsky, the chief executive of Johnson & Johnson, has also said that Covid-19 shotswill probably need to be administered annually, just like the seasonal flu shot.

Dr. Anthony S. Fauci, the head of the National Institute of Allergy and Infectious Diseases, has said that instead of a new vaccine against variants, a third dose of the Pfizer-BioNTech or Moderna vaccines in six months to a year may be just as effective. That would keep antibody levels high in each recipient, overwhelming any variant.

“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 earlier this month.

A rally against coronavirus restrictions in Woodland Hills, Calif. last May.
Mark Ralston/Agence France-Presse — Getty Images

If more evidence were needed that even a pandemic can be partisan, surveys have found that while two-thirds of Democrats report having had at least one dose of a coronavirus vaccine, two-fifths of Republicans say they do not plan to receive any.

Just over half of American adults have now received at least one shot, according to a Monmouth University poll released Wednesday.

But vaccination rates were far from even across party lines.

That suggests that President Biden has yet to succeed in his effort to depoliticize the vaccines. And it raises the question of whether the country will be able to achieve herd immunity without a stronger push from Republican leaders to bring their voters on board.

The results of the Monmouth poll lined up with those of a separate survey by Quinnipiac University, also released on Wednesday, that found 45 percent of Republicans saying they did not plan to get vaccinated.

When it comes to confronting the pandemic, Americans give positive marks to the president and to their state’s governor, but they don’t have as much faith in one another. Just 43 percent said the public had done a good job dealing with the outbreak. Democrats in particular were disappointed in their fellow citizens.

With public health experts warning that there could be another surge in Covid-19 cases if the economy reopens too swiftly, the Quinnipiac poll found that 85 percent of Democrats said they were worried about another outbreak. Just 32 percent of Republicans shared the concern.

Republicans also reported feeling much more comfortable attending large public events.

Vehicles lined up for people to receive vaccines in Loudon, N.H., last month.
Geoff Forester/The Concord Monitor, via Associated Press

Gov. Chris Sununu on Thursday said he was ending New Hampshire’s statewide mask mandate, despite calls from federal officials to keep such measures in place.

Mr. Sununu pointed to falling Covid-related deaths and the availability of vaccines as key data points for his decision, and said he would continue to monitor both.

“A state mandate does not have to be in place for us to simply know that wearing a face covering when we’re unable to maintain social distancing is just a good idea,” Mr. Sununu said at a news conference Thursday. “It’s recommended by public health, we’re strongly behind that message, and we encourage everyone to do so.”

The mask law expires Friday.

President Biden and Dr. Rochelle Walensky, the director of the Centers for Disease Control and Prevention, have called on governors and mayors to retain or reinstate mask mandates to help curb a possible new surge of the pandemic. Mr. Biden has urged all Americans to “mask up” for the first 100 days of his presidency, which comes at the end of April.

Mr. Sununu said that the state would continue to encourage social distancing and mask wearing “when appropriate” and that “that message will not change whatsoever.”

“What does change,” he said, “is the government’s imposed requirement to do so.”

In March the governor relaxed other restrictions, allowing retail businesses to operate at 100 percent capacity. The lifting of the mask mandate does not prohibit private businesses, cities or towns from enforcing mask wearing.

According to a New York Times database, coronavirus cases in New Hampshire have increased 15 percent over the past two weeks, averaging 435 new cases a day. The state is averaging 124 hospitalizations, a 41 percent increase over the past two weeks. Twenty-six percent of state residents are fully vaccinated and 54 percent have received at least one dose.

Mr. Sununu said although hospitalization rates have not dropped in the same way that infection and death rates have, “we don’t have the concerns that we had before about the health care system being overrun.”

New Hampshire’s mask mandate was originally put in place in November to protect vulnerable communities, the majority of which have since been vaccinated, Mr. Sununu said.

“It really worked, I could imagine these numbers could have been a lot, lot worse without those additional restrictions in place,” he said, adding that it “gives everyone a good pause and a breath that we are coming out of this very, very quickly.”

global roundup

Outside a vaccination center in Hong Kong earlier this month. Bookings have increased after the government brought in “vaccine bubble” measures.
Vincent Yu/Associated Press

The Hong Kong government said on Thursday that the city’s Covid-19 vaccination program, which has been hampered by low participation, would be expanded to include residents as young as 16.

Under the expanded plan, shots of the Chinese-made Sinovac vaccine will be made available to residents 18 and older, while those 16 and over will be eligible to receive the Pfizer-BioNTech vaccine.

Hong Kong has handled the pandemic far better than much of the world, with 209 coronavirus-related deaths recorded in the city of 7.5 million. Hong Kong’s chief executive, Carrie Lam, announced last week that a fourth wave of infections in the city had been brought under control.

But vaccinations have lagged: Less than 10 percent of the population has received a dose since the authorities began the vaccine rollout in February. (Distribution of the Pfizer-BioNTech vaccine was suspended for nearly two weeks over packaging defects before resuming at the beginning of April.)

Last week, Mrs. Lam unveiled a series of “vaccine bubble” measures that would lift some social distancing restrictions for residents who have been inoculated, and for businesses with vaccinated staff. The measures would allow participating bars and pubs that have been closed since November to reopen, and restaurants to expand seating.

Bookings for vaccination appointments have also sharply increased. But the plans have been criticized for being overly complicated and — until the expansion of vaccinations to include people ages 18 to 29 — discriminatory against young people.

In other news around the world:

  • Cambodia’s capital city went into a two-week lockdown on Thursday as the prime minister, Hun Sen, warned that the country was “on the verge of an awful tragedy.” After recording fewer than 500 coronavirus cases in 2020, Cambodia has had more than 4,300 in the last two months, in an outbreak traced to a foreign resident who violated quarantine and visited a nightclub. Last week, the government shut down Angkor Wat, its most popular tourist destination, for two weeks. Thirty-six people have died of Covid-19, but experts warn that the health system is ill equipped for a surge of patients.

  • In Britain, the local authorities in London and in Smethwick near Birmingham in central England have tested hundreds of thousands of people this week after finding dozens of cases of the B.1.351 variant first detected in South Africa. Britain has managed to contain the spread of this variant better than other European countries, like France, but experts have warned that new outbreaks could hamper the lifting of lockdown restrictions. Shops, hairdressers, and outdoor dining areas at pubs and restaurants reopened this week in England.

  • Iran ordered 60 million doses of the Russian Sputnik-V vaccine on Thursday, according to the state-run news agency IRNA, citing the country’s ambassador to Russia. The worst-hit country in the Middle East, Iran has recorded more than 65,000 Covid-19 deaths, although the true figure is estimated to be much higher. It received its first shipment of Sputnik-V vaccine doses in February, and Iran’s ambassador to Russia, Kazem Jalali, said the first of the 60 million new doses would be delivered in June. Iran, which has a population of more than 80 million, has administered less than 500,000 doses, according to a New York Times database.

  • Poland began inoculations against Covid-19 with Johnson & Johnson’s single-shot vaccine on Thursday, with health officials saying that any potential risks were tiny and far less serious than the health hazards posed by the coronavirus. Although Johnson & Johnson said earlier this week that it was delaying the rollout of its vaccine in Europe, Poland received its first delivery on Wednesday, and went ahead with its first injections a day later. Speaking in Warsaw on Thursday, Grzegorz Cessak, the head of Poland’s medicines regulator, described the blood clotting cases found in a small number of recipients of the vaccine as “ultra-rare,” noting that the European Union, of which Poland is a member, had so far imposed no restrictions on its use. The bloc’s top drug regulator said it would issue recommendations on the use of the vaccine next week.

Andrew Higgins, Isabella Kwai and Elian Peltier contributed reporting.

The 2020 Olympic Games are set to begin in late July after a year’s postponement because of the pandemic.
Issei Kato/Reuters

A leading member of Japan’s governing party said on Thursday that the country would consider canceling the Tokyo Olympics if rising coronavirus cases were not brought under control.

But as his comments lit up the internet, he quickly walked them back, issuing a statement saying that he had been speaking hypothetically and that “our party has not changed its support for holding a safe and worry-free Games.”

Yet the comments were the first public indication that the government is considering canceling the Games, in the face of widespread public discontent about their organization and growing concerns about the pandemic. (Polls indicate that more than 70 percent of Japanese believe the Games should be delayed again or called off entirely.)

Infection rates in Japan, although still relatively low, have climbed in recent weeks, raising fears that the country could soon face a “fourth wave” of cases as it prepares for the Games. They are set to begin in late July, a year after they were postponed because of the pandemic.

Speaking to a television news program, Toshihiro Nikai, the secretary general of the governing Liberal Democratic Party and one of the most powerful politicians in Japan, said that “if the situation becomes more difficult, we’ll have to completely cancel” the Olympics, according to local media reports. The interview was prerecorded and has not yet aired.

Hours later, as headlines about his comments were splashed across the news media, Mr. Nikai sought to soften the message with a statement reaffirming his commitment to the Games.

So far, Japan has avoided the worst of the pandemic, recording fewer than 10,000 deaths — an achievement that many attribute to ubiquitous mask-wearing and an effective public health campaign.

But the country has been slow to roll out vaccines, with shots for elderly citizens only beginning earlier this month.

In the last several weeks, newer and more contagious variants of the coronavirus have driven up case counts in major cities, prompting tough restrictions in parts of Tokyo and other municipalities. Experts are concerned that the Olympics, which are expected to welcome thousands of athletes from more than 200 countries, could become a superspreading event.

The government has reiterated that it intends to put on “safe” Games as a symbol of national and global resilience, although in a modified form that bans, among other things, spectators from abroad.

Throughout the pandemic, a greater percentage of white parents than nonwhite parents have expressed a preference for in-person learning for their children. Nonwhite communities have been harder hit by the virus, and parents of color often describe having little trust in their local public schools.

But now a new study offers another theory: Much of the racial disparity, a researcher says, can be explained by the fact that schools serving large numbers of nonwhite children have been those most likely to remain closed, regardless of local virus rates.

According to the study, which is based on regular surveys of about 1,500 parents conducted between November and March, extended closures may send the message that schools are unsafe, leading parents to conclude that their children should stay home.

But when districts announced they were reopening, parents of all races became more enthusiastic about in-person learning.

The study comes as some large districts continue to wrestle with how to reopen, especially for older students. In Chicago, the school district and teachers’ union reached a tentative agreement on Thursday to reopen high schools next week for the first time in over a year. But many parents in the city, disproportionately Black and Latino, continue to choose remote learning.

The new study, by Vladimir Kogan, a political scientist at Ohio State University, suggests that both nonwhite and white parents’ fears of Covid-19, as well as their personal experiences with the virus, were less of a factor in their schooling preferences than were the decisions made — and messages sent — by local education officials.

Of those parents whose children’s schools were closed in November and who preferred remote learning then, 88 percent still preferred it in February if their schools remained closed. But, the researchers found, that number dropped to 57 percent among those whose schools had reopened.

The second-most important predictor of parents’ preferences was political outlook, the study found. Those who said they trusted President Donald J. Trump were more likely to prefer in-person schooling than those who said they trusted President Biden.

The longest school closures have occurred in cities, like Chicago, where powerful teachers’ unions have often resisted returning to in-person learning. Unions have cited concerns such as slow vaccine rollouts, poor ventilation in school buildings and lack of space for physical distancing. Urban schools serve a disproportionate number of nonwhite students.

About 60 percent of American children now attend schools that offer daily, in-person classes. But millions of parents continue to choose to keep their children home — a number that could be reduced, Professor Kogan said, if educators and school districts sent a positive message to parents about safety and the importance of in-person learning and socialization.

“It’s not acceptable that we have a two-tier education system where white kids go to school in person disproportionately and students of color disproportionately go to school online,” he said.

The paper was published by the American Enterprise Institute, a center-right think tank. It is based on data from the Understanding America Study, a survey conducted by the University of Southern California.

After trying to reopen without coronavirus cases soaring, France went back into lockdown last month.
Andrea Mantovani for The New York Times

France has now recorded 100,000 coronavirus deaths, according to figures released on Thursday, as the country struggles to find a path out of nationwide restrictions that have done little to curb a third wave of infections.

France is the third European country to surpass 100,000 deaths, after Britain and Italy, and is the eighth to do so worldwide. Its approach to the pandemic has been beset by a succession of missteps — in delivering masks last spring, then facing a shortage of tests, and in recent months, struggling to ramp up its vaccination campaign.

A deep sense of fatigue and frustration has set in over a seemingly endless cycle of restrictions in the country, which entered a third national lockdown last month.

“Although all our energy is now focused on getting through this ordeal, we will not forget any face, any name,” President Emmanuel Macron wrote on Twitter about the 100,000 people who have died.

Though not as strict as the first lockdown last spring, which turned Paris into a ghost town, the latest restrictions limit outdoor activities, close nonessential shops, ban travel between regions and shut schools for a month. An overnight nationwide curfew has been in force since mid-December.

France has reported an average of 35,000 new cases a day over the past two weeks — well below the 50,000-a-day pace of last fall, but still enough to strain the country’s overstretched hospitals. More than 5,900 patients were in intensive care beds this week, the highest number since April last year.

The country’s vaccination campaign, which has been hampered for months by logistical issues and challenges over the use of the AstraZeneca vaccine, gathered speed this week after the authorities opened mass vaccination centers, including one in France’s largest stadium.

Fewer than 12 million people — about 18 percent of the total population — have received a first dose so far, putting France far behind countries like the United States, Britain and Israel. Mr. Macron’s government has said it hopes to administer a total of 20 million first doses by mid-May, when it plans to reopen parts of the economy.

Montreal earlier this month. This week Canada surpassed the United States in the number of new coronavirus cases per 100,000 residents.
Graham Hughes/The Canadian Press, via Associated Press

Prime Minister Justin Trudeau of Canada has come under renewed political attacks for the country’s slow vaccination rate, which lags behind the United States’, just as Canada’s daily per capita increase in Covid-19 cases has edged past that of its neighbor for the first time.

The latest wave of coronavirus infections in Canada, driven largely by the B.1.1.7 variant, has strained the capacity of intensive care units in many parts of the country. Children’s hospitals in Toronto and Ottawa, Ontario’s two largest cities, took the unusual step of opening their intensive care beds to adult patients this week.

Restrictions have been reimposed or expanded in many provinces, with a nightly curfew in parts of Quebec.

For a country that had prided itself on its response to the pandemic, the reversal has come as a blow, if a symbolic one. In Canada, the seven-day average of new daily cases is now at 23 for every 100,000 people, compared with 22 in the United States, according to a New York Times database.

Mr. Trudeau was challenged on Wednesday by opponents from both ends of the political spectrum after the health authorities in the Scarborough area of Toronto closed two clinics and canceled 10,000 vaccinations that had been scheduled for this week because of delayed shipments of the Moderna vaccine.

“The Liberal government has failed to secure enough doses and Canadians are angry,” Jagmeet Singh, leader of the center-left New Democratic Party, said in the House of Commons.

“Canadians deserve better,” said Michelle Rempel Garner, a Conservative lawmaker. “There are zoo animals being vaccinated in the United States.” (She may have been referring to apes at the San Diego Zoo that received a coronavirus vaccine last month.)

Mr. Trudeau acknowledged that the situation was deteriorating in some provinces, but he argued that vaccine shipments to Canada had increased substantially in recent weeks. Canada, which relies entirely on imports for its vaccines, has delayed second shots by up to four months to increase the number of people who will have received at least one inoculation.

Mr. Trudeau said that among the Group of 7 countries, Canada ranks third after the United States and Britain for the number of citizens who have been given at least one injection. More than 15 percent of Canadians have received one shot, compared with 37 percent in the United States and 48 percent in Britain.

Yet Canada’s full vaccination rate lags even more: 2.3 percent of Canadians have been fully vaccinated, compared with 23 percent of Americans and 12 percent of Britons.

Coronavirus cases have risen sharply in recent weeks in the Miami area. Crowds of people walked along Ocean Drive in Miami Beach last month.
Joe Raedle/Getty Images

Swarms of spring-break revelers descended on the Miami area in February and March, lured by the promise of loose Covid-19 regulations. Now Miami-Dade County has a spike in coronavirus cases on its hands.

The county is reporting an average of 1,450 new cases a day, 27 percent more than the average two weeks ago, according to a New York Times database.

The virus is spreading in Miami-Dade at more than twice the national rate. The county has been adding an average of 53 new cases a day for each 100,000 residents, far more than many other large metropolitan areas.

Several factors could be contributing to the surge, according to Mary Jo Trepka, an epidemiologist at the Robert Stempel College of Public Health at Florida International University.

For one thing, Dr. Trepka said, many of the visitors who flocked to the Miami area from across the country gathered in large crowds, ignoring social distancing and failing to wear masks.

For another, she said, the predominant version of the virus now circulating in South Florida seems to be B.1.1.7, the more contagious and possibly more lethal virus variant first identified in Britain. The variant became the most dominant one across the country, the Centers for Disease Control and Prevention said last week.

And then there were Passover and Easter, when many local residents congregated with family members.

“It’s a combination,” Dr. Trepka said, of “low vaccination rates; second, the variants; and third, the behavior — the lack of physical distancing, the lack of mask-wearing.”

She noted that the recent increase in cases was concentrated in people under the age of 65. “The majority of younger people have not been vaccinated, so they are susceptible,” she said, adding that now that everyone over 16 is eligible in Florida, more young people should get a shot as soon as they can.

The county reported on Wednesday that more than a million residents had received at least one dose of vaccine. Statewide, about 36 percent of residents have received at least one dose and about 22 percent are fully vaccinated.

Dr. Trepka said it was also important for people to go on maintaining social distance and wearing masks. “I really think, at this point, everybody needs to do their part,” she said.

Miami Beach imposed a temporary 8 p.m. curfew in the city’s South Beach entertainment district in late March, but it was widely ignored by young revelers.

“It looked like a rock concert,” Raul J. Aguila, the interim city manager, told reporters at the time. “You couldn’t see pavement, and you couldn’t see grass.”

Mayor Daniella Levine Cava of Miami-Dade County lifted a countywide midnight curfew on Monday that had been in place since September, though she encouraged residents to keep taking precautions. “We can see the light at the end of the tunnel, brighter than ever before,” she wrote on Twitter. “Let’s not go backward when we’ve come so far.”

Florida’s governor, Ron DeSantis, was slow in locking down the state last year, and far earlier than most other governors, he pledged that schools would open in the fall and life would start returning to normal.

Dr. Trepka said the increase in cases “looks to be our fourth wave,” and added, “How high will it get? I don’t know.”

People waiting in line for coronavirus tests in Corona, Queens, last month.
James Estrin/The New York Times

In January 2020, just weeks after the first Covid-19 cases emerged in China, the full genome of the new coronavirus was published online. This genomic sequence is how scientists scrambled to come up with diagnostic tests for the virus. But since last year, the virus has mutated and the coronavirus has evolved.

The emergence of new variants has sparked a flurry of interest in developing tests for specific viral mutations and prompted concerns about the accuracy of some existing tests. The Food and Drug Administration has warned that new mutations in the coronavirus could render some tests less effective. Last week, PATH, a global health nonprofit, launched two online dashboards to monitor how certain variants might affect the performance of diagnostic tests.

“With these Covid diagnostics, we were on a time crunch, we had to get something out there,” said Lorraine Lillis, the scientific program officer at PATH. “Diagnostics take a long, long time, and we’d normally challenge them with multiple variants.” Now, she added, they are doing that, “but in real time.”

So far, scientists agree that there is no evidence that the known variants of concern are causing tests to fail completely. “The tests today work very, very well,” said Mara Aspinall, an expert in biomedical diagnostics at Arizona State University.

But manufacturers and regulators will need to remain vigilant to ensure they keep pace with a constantly changing virus, scientists say. If variants begin to evade detection, that could be consequential not only for individual patients, who may not receive the treatment they need, but also for the broader population.

If a test misses someone who is infected by a variant, then that person may not realize they need to isolate and could unknowingly spread that variant to others, said Gary Schoolnik, a physician and infectious disease expert at Stanford University. “And that’s how a diagnostic test, if it’s missing variants, can actually promote the spread of that variant.”

Migrant workers waited for trains in Mumbai, India, on Monday. The city is under lockdown because of the latest surge in Covid-19 cases.
Atul Loke for The New York Times

As dawn broke, Kaleem Ansari waited outside the central rail station in Mumbai, India, with thousands of others for his train to pull in. Mr. Ansari, who works in a sandal factory, carried old clothes in his backpack and 200 rupees — not quite $3 — in his pocket.

Mumbai was locking down as a second wave of the coronavirus rippled through India. Mr. Ansari, from a small village nearly a thousand miles away, stayed in the city for the first lockdown, in 2020, and vowed not to suffer through another.

“I remember what happened last time,” he said on Wednesday. “I just have to get out of here.”

As India’s cities shut down and workers head to rural areas, health experts fear the virus could devastate poorly equipped villages, as it did before. The country hit another daily case record on Thursday, reporting more than 200,000 new infections, and transit hubs are packed with fleeing workers.

The traumatic mass exodus last year, after one of the world’s toughest national lockdowns eliminated millions of jobs, fueled the most disruptive migration in the region since the partition of 1947. Tens of millions of low-paid migrant workers fled, some on blistered feet, to reach loved ones and places where they could afford to live.

Hundreds died on the way. Even more died back home. The migration also spread the virus, swamping remote districts with the sick.

This time, the lockdown does not extend nationwide, but cities are increasingly enforcing restrictions, and the migrant exodus is likely to grow.

On Tuesday night, for example, Maharashtra State, which includes Mumbai, banned public gatherings and closed most businesses for two and a half weeks. The authorities had little choice, experts say. New infections are exceeding the heights of the first wave, and the true number is likely to be many times higher, while testing rates per capita lag Western countries. (So far, only about 8 percent of Indians have been vaccinated.)

The death rate, while lower than in the United States and elsewhere, is rising.

Last year, to protect uninfected villagers, officials in the large eastern state of Bihar intercepted arriving migrants, screened them and sent them to quarantine centers whether they had symptoms or not. This time, migrants from cities like Mumbai — where the positivity rate recently hit 30 percent — are simply stepping off buses and walking into hometowns, said Nafees Ahmad Sheikh, a cafe worker who left Mumbai last week, and two other recent arrivals.

“The rich can deal with another lockdown, but what will the poor do?” Mr. Sheikh said. He said he would rather die in his home village than in a city “that treats us like disposable items.”

Some officials said towns were requiring temperature checks and running quarantine centers, but one official said few centers were functioning because many contractors were not paid last year.

As for Mr. Ansari, he just wants to get home. Last time public transport had shut down and he ran out of money. He said the police would beat him when he ventured out for food. At one point he was eating only one small bowl of rice a day, he said, and feared he would starve.

“Nobody cares about us,” he said, “either here or there.”

Waiting to receive a vaccination in Vieques, Puerto Rico, last month.
Carlos Giusti/Associated Press

Puerto Rico is reporting a sharp rise in new coronavirus cases and hospitalizations amid a lagging vaccine rollout, and officials are worried that a combination of new variants and people neglecting basic pandemic safety measures may be making things worse.

The island is reporting an average of 1,019 new cases a day — a jump from just 211 a month ago, according to a New York Times database. Hospitalizations have spiked 91 percent in the past two weeks, and deaths are on the rise again.

Like much of the United States, Puerto Rico started to report a drop in cases in mid-January, but in the spring it began to reverse course.

At a news briefing on Monday, Puerto Rico’s health secretary, Carlos Mellado, urged people to change their behavior. More than half the cases, he said, had been traced to family activities.

“I think that there’s a citizen responsibility here,” he said in Spanish. “Every person has to empower themselves for their own health.”

Asked whether he would support a lockdown, Mr. Mellado said, “I won’t discount anything.”

Last week, Puerto Rico announced that it would temporarily close schools again, a month after some were allowed to reopen for the first time in a year. Gov. Pedro Pierluisi also issued an executive order that went into effect last week extending Puerto Rico’s overnight curfew, prohibiting certain mass gatherings and ordering commercial businesses and restaurants to close at 9 p.m.

“Faced with an uptick in cases like we’re seeing, my responsibility is to act immediately,” Mr. Pierluisi said.

Mr. Mellado said many people didn’t seem to understand the severity of the situation, noting that more contagious variants may be contributing to the spread of the virus and that even people who have been fully vaccinated shouldn’t flout the safety protocols.

Residents and experts have also expressed concern about the increasing tourism to the island, with many visitors seemingly ignoring virus precautions. Puerto Rican business owners told CNN last month that visitors often didn’t follow mask-wearing rules or respect the curfew. And recent viral videos have highlighted the issue, with one showing dozens of people crowded together, dancing and singing without masks at a popular San Juan plaza.

Puerto Rico has been slow to vaccinate, lagging behind most states and U.S. territories. About 26 percent of residents has received at least one dose of a Covid-19 vaccine, including about 16 percent who are fully vaccinated. Puerto Rico made anyone 16 or older eligible for a vaccine on Monday.

For Puerto Rico, the pandemic has been yet another crisis. The island has also had to contend with a devastating hurricane, a political crisis and a series of earthquakes in recent years. The Biden administration recently announced that it would release $1.3 billion in delayed aid to the territory to protect against future climate disasters and remove restrictions on another $4.9 billion.