C.D.C. Panel Keeps Pause on Use of J&J Vaccine, Weighing Risks

“Right now, we believe these events to be extremely rare, but we are also not yet certain we have heard about all possible cases, as this syndrome may not be easily recognized as one associated with the vaccine,” Dr. Rochelle P. Walensky, the C.D.C. director, said at a White House news conference on the pandemic on Wednesday.

During the panel discussion, experts noted that the “risk window” for the condition among vaccine recipients was still open and that new cases might emerge, because nearly 3.8 million people had received the shot within the last two weeks. In the six women, the severe clotting developed within about two weeks of the shot.

Other experts encouraged dissemination of health information on diagnosis and treatment of the condition so that awareness would spread among doctors, emergency rooms and people who had received the vaccine. A key point is that the blood-thinner heparin, a common treatment for clots, can harm these patients and should not be used.

Officials also noted that people with the condition needed to be treated as soon as possible because the clots were so serious. Some patients needed invasive procedures to remove large clots from blood vessels in their brains.

Several panel members reiterated that two other vaccines — from Moderna and Pfizer-BioNTech — are available, neither associated with the clotting problem, so continuing the pause would not stop most people in the United States from being vaccinated.

At the news conference, Jeffrey D. Zients, the White House’s pandemic coordinator, said that the pause would not generally interrupt the momentum of the country’s vaccination campaign.

“In the very short term, we do expect some impact on daily averages as sites and appointments transition from Johnson & Johnson to Moderna and Pfizer vaccines,” he said. “We have more than enough Pfizer and Moderna vaccine supply to continue or even accelerate the current pace of vaccinations.”

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Changing Strategy, the E.U. Bets Big on Pfizer to Battle Covid

BRUSSELS — Bruised by major disruptions in supplies of the AstraZeneca and Johnson & Johnson vaccines, the European Union Wednesday announced it was putting trust and money into the Pfizer-BioNTech shot to salvage its vaccination rollout and secure doses for the future.

The pivot away from AstraZeneca, once a pillar of the E.U. inoculation program, comes after months of discord over delayed shipments and as the company battles worries over rare potential side effects of its shots.

In announcing the change in strategy, Ursula von der Leyen, president of the European Commission, said Pfizer had agreed to an early shipment of doses that she said should likely allow the bloc to reach its goal of inoculating 70 percent of adults by the end of the summer.

That goal was in jeopardy after AstraZeneca failed to deliver on expected doses in the first quarter of the year, then suffered fresh setbacks over potential side effects related to blood clots. The European vaccine campaign was dealt a further blow Tuesday when Johnson & Johnson said it would delay its own rollout in Europe because of similar concerns and after regulators paused its use in the United States.

supply disruptions from AstraZeneca in late January, and then with the emergence of the potential rare blood disorder that has battered the public’s confidence in vaccines and led to appointment cancellations.

“As we can see with the announcement by Johnson & Johnson yesterday, there are still many factors that can disrupt the planned delivery schedules of vaccines,” Ms. von der Leyen said Wednesday.

Ms. von der Leyen said the Pfizer doses under negotiation for the next two years would include potential booster shots to extend the immunity of people who have already been inoculated, as well as possible new shots or boosters targeting emerging variants that might prove resilient against existing vaccines.

The AstraZeneca and Johnson & Johnson vaccines performed well in clinical trials and the possible dangerous side effects have been rare. But trials of the Pfizer and Moderna shots shows that they were even more effective in preventing infection, and similar side effects have not emerged. Another mRNA vaccine, from CureVac, is in clinical trials.

On Wednesday, the European Medicines Agency, the bloc’s top drug regulator, said it was expediting its investigation of “very rare cases of unusual blood clots” in recipients of the Johnson & Johnson vaccine, and expected to issue a recommendation next week. While the evaluation is ongoing, the agency reiterated its view that the benefits of the vaccine outweigh the risks.

In a setback for AstraZeneca, Denmark on Wednesday became the first country to permanently stop the administration of the company’s vaccine, saying the potential side effects were significant enough to do so given that it had the pandemic under control and could rely on the Pfizer and Moderna inoculations.

With the fresh commitment by Pfizer to bring forward the delivery of 50 million doses originally slated for the end of the year, the company expects to deliver 250 million doses in total to the bloc by the end of June.

Ms. von der Leyen said more than 100 million people in the European Union had already received at least one vaccine dose, and 27 million had received both. The additional Pfizer vaccines, together with 35 million doses expected from Moderna over the next three months, and a more limited use of AstraZeneca doses already in the pipeline, should likely be enough to get the bloc to the coveted milestone of reaching 255 million people by September, E.U. officials said.

In stark contrast to the criticism of AstraZeneca’s handling of its E.U. dealings, Ms. von der Leyen praised Pfizer effusively, highlighting how important the company’s ability to respond quickly to help the European Union has been.

“I want to thank BioNTech/Pfizer; it has proven to be a reliable partner,” Ms. von der Leyen said. “It has delivered on its commitments, and it is responsive to our needs.”

Addressing another sore point, Ms. von der Leyen said that the future Pfizer doses would be produced in the European Union.

Ample exports from the factories within the bloc to the rest of the world have enabled countries like Mexico and Canada to launch their vaccination campaigns, but those exports have also been identified as one reason there weren’t enough vaccines to go around in Europe.

The United States and Britain, by contrast, held tight to the vaccines made in their countries, helping speed along their inoculation efforts.

Monika Pronczukcontributed reporting.

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Western Warnings Tarnish Vaccines the World Badly Needs

The solution in many European countries — to stop using seemingly riskier vaccines in younger people, who are at lower risk from Covid-19 — would be unworkable in Africa, where the median age in many countries is below 20.

And any further restrictions would compound the hurdles facing Covax, among them a paucity of funding for every part of inoculation programs beyond the touchdown of doses at airports.

Mali, in western Africa, has administered 7 percent of the AstraZeneca doses that Covax has delivered. Sudan, in eastern Africa, has given 8 percent of the doses it has received.

Skittishness over the AstraZeneca and Johnson & Johnson vaccines, analysts fear, could stoke demand for Russian- and Chinese-made shots about which far less is known. As it is, some global health officials have turned their attention to the Novavax vaccine, which is not yet authorized but makes up a third of Covax’s portfolio.

“Even at this stage of the pandemic, we have our fingers crossed that some vaccine will work to help vaccinate developing countries, instead of ramping up production of vaccines we know work,” said Zain Rizvi, an expert on medicines access at Public Citizen, an advocacy group.

In Kenya, where enthusiasm for vaccines is high in cities but perilously low in rural areas, “the story about blood clots from Europe could not have come at a worse time,” said Catherine Kyobutungi, the director of the African Population and Health Research Center there. “Even those who were perhaps on the fence, and leaning toward getting vaccinated, all of a sudden had second thoughts,” she said.

The American pause on Johnson & Johnson shots promised a second media furor.

“When the F.D.A. suspends, it makes headlines for days,” she said. “When it lifts the suspension, it doesn’t make as many headlines.”

Mady Camara contributed reporting from Dakar, Senegal.

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Denmark says it’s permanently stopping use of the AstraZeneca vaccine.

Denmark on Wednesday became the first country to plan to permanently stop administering the AstraZeneca vaccine, a month after suspending its use following reports that a small number of recipients had developed a rare but serious blood-clotting disorder.

The director general of the country’s health authority, Soeren Brostroem, said Denmark was able to halt use of the vaccine because it had the pandemic under control and could rely on two other vaccines, from Pfizer and Moderna.

The Danish announcement is another setback for the AstraZeneca shot, which is easy to store and relatively cheap, and was expected to be the foundation of vaccination campaigns around the world.

The country initially suspended the use of the vaccine on March 11, along with Iceland and Norway. Several other European countries, including France, Germany and Italy, followed suit last month.

later recommended that countries keep using the vaccine, saying its benefits far outweighed any potential risks for most people.

Last week, though, the European regulator listed blood clots as a potential very rare side effect of the vaccine.

Several countries that had paused and restarted use of the vaccine have since said they would stop using it in younger people. Britain, which has administered around 20 million AstraZeneca doses, said it would offer alternative vaccines to people under 30.

“Based on the scientific findings, our overall assessment is there is a real risk of severe side effects associated with using the Covid-19 vaccine from AstraZeneca,” Dr. Brostroem, the Danish health official, said in a statement. “We have, therefore, decided to remove the vaccine from our vaccination program.”

“If Denmark were in a completely different situation and in the midst of a violent third outbreak, for example, and a health care system under pressure,” he added, “then I would not hesitate to use the vaccine, even if there were rare but severe complications associated with using it.”

Danish health officials said that they might reintroduce the AstraZeneca vaccine “if the situation changes.”

Public health officials have warned that pausing administration of vaccines like AstraZeneca’s or Johnson & Johnson’s could do more harm than good. They note that among seven million people vaccinated with the single-dose Johnson & Johnson vaccine in the United States, six women had developed the rare blood clots — fewer than one in one million. It is not yet known whether the vaccine had anything to do with the clots, but even if it did, the risk is smaller than that of getting struck by lightning in a given year (one in 500,000).

Denmark, which has a population of 5.8 million, has managed to contain the pandemic better than its neighbor Sweden or many other European countries. As of Wednesday, Denmark had recorded 2,447 Covid-related deaths.

Almost one million people in the country have received at least a first dose of a vaccine, 77 percent of them the one from Pfizer, according to Denmark’s Serum Institute. Around 15 percent received a first dose of the AstraZeneca vaccine before the authorities suspended its use last month, and the remaining 8 percent received the Moderna vaccine.

The country’s health authorities said that people who received a first dose of the AstraZeneca vaccine would be offered a different vaccine for their second dose.

Jasmina Nielsen contributed reporting.

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

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

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

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

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

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

months of short supplies and logistical problems.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Benjamin Mueller contributed reporting.

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My Family’s Global Vaccine Journey

On Feb. 22, Mom texted that she and Dad had booked a March 11 appointment to get their first shots, followed by second doses in April. A day later, she reported that Dad hadn’t pressed the button to confirm the appointment on the online booking system and had lost the slots.

The next week, they texted again: They had walked to a private clinic that was dispensing Sinovac shots. After a short wait, they received the vaccine. On April 2, they told us that they had gotten their second dose of Sinovac and were feeling fine. Mom groused that even though they had an appointment, they “still need to wait for half an hour.”

Our responses were more enthusiastic.

“Great news,” I wrote.

“Yay!” Pui-Ying texted, followed by celebratory emojis.

“Congrats!” Pui Ling said.

Pui-Ying had moved with her family to Malawi in 2016 to work as a doctor and conduct clinical research on children’s health. Resources at the Queen Elizabeth Central Hospital, where she works, were limited. When Madonna’s charity helped finance the construction of a new children’s wing at the hospital, which opened in 2017, it was big news.

Staffing was tight even before the coronavirus, Pui-Ying said. When the pandemic came, the hospital decided on a one-week-on, one-week-off routine to reduce staff exposure to Covid-19 while ensuring that enough medical professionals would be working at all times. Masks, gloves and other protective equipment were scarce.

In pediatrics, Pui-Ying and her colleagues set up a “respiratory zone” for children with Covid-19. It was essentially a two-room ward, with about a dozen beds in the main room. The second room, which was an isolation unit, had space for four children.

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Blood Clots Linked to AZ Vaccine Stem From Rare Antibody Reaction

Two reports published on Friday in a leading medical journal help to explain how AstraZeneca’s Covid vaccine can, in rare cases, cause serious and sometimes fatal blood clots.

Scientific teams from Germany and Norway found that people who developed the clots after vaccination had produced antibodies that activated their platelets, a blood component involved in clotting. The new reports add extensive details to what the researchers have already stated publicly about the blood disorder.

Why the rare reaction occurred is not known. Younger people appear more susceptible than older ones, but researchers say no pre-existing health conditions are known to predispose people to the problem, so there is no way to tell if an individual is at high risk.

Reports of the clots have already led a number of countries to limit AstraZeneca’s vaccine to older people, or to stop using it entirely. The cases have dealt a crushing blow to global efforts to halt the pandemic, because the AstraZeneca shot — easy to store and relatively cheap — has been a mainstay of vaccination programs in more than 100 countries.

statement on its website, AstraZeneca said it was “actively collaborating with the regulators to implement these changes to the product information and is already working to understand the individual cases, epidemiology and possible mechanisms that could explain these extremely rare events.”

The two new reports were published by The New England Journal of Medicine. One from Germany describes 11 patients, including nine women ages 22 to 49. Five to 16 days after vaccination, they were found to have one or more clots. Nine had cerebral venous thrombosis, a clot blocking a vein that drains blood from the brain. Some had clots in their lungs, abdomen or other areas. Six of the 11 died, one from a brain hemorrhage.

One patient had pre-existing conditions that affected clotting, but during a news briefing on Friday, Dr. Andreas Greinacher, an author of the report, said those conditions most likely played only a minor role in the disorder that occurred after vaccination.

second report, from Norway, described five patients, one male and four female health care workers ages 32 to 54, who had clots and bleeding from seven to 10 days after receiving the AstraZeneca vaccine. Four had severe clots in the brain, and three died. Severe headaches were among their early symptoms. Like the German patients, all had high levels of antibodies that could activate platelets.

The team from Norway also recommended treatment with intravenous immune globulin. The researchers said the disorder was rare, but “a new phenomenon with devastating effects for otherwise healthy young adults,” and they suggested that it may be more common than previous studies of the AstraZeneca vaccine had indicated.

On Friday, European regulators also said they were reviewing reports of a few blood clot cases that occurred in people who had received the Johnson and Johnson vaccine. In the United States, federal agencies are investigating reports of a different type of unusual blood disorder involving a precipitous drop in platelets that emerged in a few people who had received either the Pfizer-BioNTech or Moderna vaccines.

Benjamin Mueller and Melissa Eddy contributed.

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AstraZeneca Vaccine Faces New Setbacks in U.K. and European Union

LONDON — Britain said on Wednesday that it would curb the use of AstraZeneca’s vaccine in adults under 30 because of the risk of rare blood clots, a blow to the efforts of scores of countries reliant on the vaccine to stamp out the coronavirus pandemic amid a global surge in cases.

Adding to the unease, the European Medicines Agency outlined a “possible link” between the vaccine and rare clots, even as it said that Covid-19 remained the far greater threat, leaving decisions about how to use the vaccine in the hands of the 27 member states of the European Union.

Taken together, the decisions represented a considerable setback for the AstraZeneca shot, which has been seen as the principal weapon in the battle to reduce deaths in the vaccine-starved global south.

The world’s most widely administered coronavirus vaccine, it is far less expensive and easier to store than some of the alternatives, spurring its use in at least 111 countries, rich and poor. AstraZeneca, based in Britain, has promised to deliver three billion doses this year, enough to inoculate nearly one in five people worldwide.

Cameroon and the Democratic Republic of Congo have already delayed injections of AstraZeneca’s vaccine amid mounting concerns in Europe. Any further hesitation, scientists said, could cost lives.

“In developing countries, the dynamic is to either use the vaccine you have, or you have nothing,” said Penny Ward, a visiting professor in pharmaceutical medicine at King’s College London. “In which case, carnage ensues.”

For the vast majority of people, British and European regulators said on Wednesday, the benefits of AstraZeneca’s shot far outweigh the risks. The clotting problems were appearing at a rate of roughly one in 100,000 recipients across Europe. Meanwhile, in Britain, the vaccine has driven down hospitalizations from Covid-19 — which can, itself, cause serious clotting problems — and saved thousands of lives, regulators said.

most people doubted the vaccine’s safety.

Over all, use of the shot has suffered: Across Europe, 64 percent of delivered doses of AstraZeneca’s vaccine have been injected into people’s arms, markedly lower than the rates for other shots.

“One hoped there would have been collaboration, and more discussion, between regulators, instead of lots of different countries going off in all sorts of directions,” Professor Ward said. “That aspect has really been the most unhelpful.”

watch for certain symptoms, including severe and persistent headaches and tiny blood spots under the skin. Doctors’ groups have circulated guidance about how to treat the disorder.

As of March 22, regulators had carried out detailed review of 86 cases, 18 of which were fatal, they said.

Concerns about the shot became acute enough in Britain this week that the University of Oxford, which developed the vaccine with AstraZeneca, stopped giving doses as part of a two-month-old trial in children.

“Safety has been our priority throughout the development of the vaccine,” Andrew Pollard, the Oxford researcher in charge of the trials, said on Wednesday. The identification of the clots, he added, “shows that the safety system works.”

In the United States, AstraZeneca is preparing to apply for emergency use authorization from the Food and Drug Administration. If and when they take up the application, that agency’s regulators are expected to scrutinize the clotting cases.

The United States, flush with vaccines from three other makers, may not ultimately need AstraZeneca’s shot. But any ruling by the F.D.A. is expected to hold considerable weight in some of the poorer nations that are relying on the shot.

The World Health Organization said a vaccine safety subcommittee had met on Wednesday and noted that “rare adverse events following immunizations should be assessed against the risk of deaths from Covid-19 disease and the potential of the vaccines to prevent infections.” It said that a link with the clotting problems, while “plausible,” had not been confirmed.

For Britain, the AstraZeneca vaccine has become a huge source of national pride, and the backbone of the country’s speedy inoculation program.

Even if younger people are at lower risk from severe Covid-19, scientists have said, inoculating them remains essential to creating enough protection in the population to end the pandemic.

Emma Bubola, Monika Pronczuk and Rebecca Robbins contributed reporting.

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