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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How to Double the Vaccination Pace

The development of the Covid-19 vaccines happened with great urgency, for obvious reasons.

One of the timesaving techniques by Moderna and Pfizer involved scheduling the two vaccine doses fairly close together — just three or four weeks apart — during the research trials. The companies did not test multiple gaps between the two shots to see which was the most effective. They each chose a short gap to finish the trials as quickly as possible.

The decision made a lot of sense. It allowed the U.S. mass vaccination program to start in December, rather than pushing it back a few months. Many lives have been saved as a result.

But the approach means that nobody knows what is the most effective gap between the two shots. Maybe it really is three to four weeks. Maybe a longer delay is just as effective (or, for that matter, even more effective).

And the short delay does come with a large downside.

The U.S. is choosing to give millions of people a second shot while making millions of others wait for their first. That’s happening even though a single shot provides a high degree of protection and even as a more severe, contagious coronavirus variant is sweeping the country. Both cases and hospitalizations have risen in recent days, and deaths have stopped declining.

are calling on the Biden administration or governors to change policy and prioritize first doses:

  • “We’ve missed a window, and people have died,” Sarah Cobey of the University of Chicago told my colleague Carl Zimmer.

  • “Getting as many people as possible a vax dose is now urgent,” Dr. Atul Gawande, the surgeon and medical writer, tweeted.

  • “We need to get more people vaccinated,” Dr. Ezekiel Emanuel of the University of Pennsylvania told me.

In a USA Today op-ed, Emanuel, Govind Persad and Dr. William Parker argue that spreading out the first and second shots would be both more equitable and more efficient. It’s more equitable because working-class, Black and Latino communities all have lower vaccination rates, which means that first shots disproportionately now go to the less privileged and second shots go to the more privileged. It’s more efficient because a delay in second shots would allow the country to double the number of people who receive a first shot in coming weeks.

Doing so could prevent other states from experiencing the current misery in Michigan, where a severe outbreak fueled by the B.1.1.7 variant has overwhelmed hospitals. In much of the South and the West, the variant is not yet as widespread.

The biggest worry about a longer delay between shots is that it may allow a new variant to develop in people while they are waiting for their second shot and do not yet have full protection. Dr. Anthony Fauci, the top Biden administration adviser, opposes a longer delay largely because of this possibility.

But it remains only a theoretical possibility, as Dr. Catherine Schuster-Bruce, a British health care writer, has noted. There is no data showing that variants are more likely to develop in people who have received only one shot, just as there is no data showing that a three- or four-week gap between shots is ideal.

There is real-world evidence — from Britain — showing large benefits from maximizing the number of people who get one shot.

total shots per capita. The difference is that Britain has deliberately delayed second shots, by up to 12 weeks. The results are impressive.

Despite being the country where the B.1.1.7 variant was first detected, Britain now has the pandemic under better control than the U.S. does. Both cases and deaths have fallen more sharply, highlighting the power of a single vaccine dose. “The levels of antibodies after the first shot are sky-high,” Dr. Robert Wachter of the University of California, San Francisco, told me.

has called the possibility that delaying second shots would lead to new variants a “real worry but quite a small real worry.”

A few weeks ago, I was concerned that changing to a different vaccination schedule might not be worth the confusion and uncertainty it could cause. But I find the latest arguments to be strong. The costs of switching are almost all hypothetical. The benefits are concrete.

President Biden and his aides are fond of saying that they “follow the science” when setting Covid policy. Their current definition of the science, however, is quite narrow. It revolves almost completely around the Moderna and Pfizer trials, which didn’t test what the ideal gap between shots was. Their definition ignores the mountain of real-world evidence about the strength of a single shot.

Are they warming the planet?

Lives Lived: The anthropologist Marshall D. Sahlins explored how individuals shape and are shaped by their cultures, a point he put in practice as the inventor of the “teach-in” against the Vietnam War. Sahlins died at 90.

said on “Fresh Air” last year.

Here’s a Times review.)

“It was so much fun, maybe more fun than I ever had in my life, because we were inventing something new with almost no resources,” Stamberg said in an interview with Next Avenue.

Read more: An excerpt from Napoli’s book tells the story of NPR’s first pledge drive.

What to Cook

pasta primavera with asparagus is a celebration of spring.

In times of uncertainty, trivia has the power to provide answers.

After Anthony Bourdain’s death, his longtime assistant was left to finish his last book. “World Travel: An Irreverent Guide” comes out next week, and it’s “an enduring embodiment of Anthony Bourdain’s love for the whole world,” Sebastian Modak writes in The Times.

The hosts got serious about police brutality.

play online.

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AstraZeneca Vaccine and Blood Clots: What Is Known So Far

Still, German researchers have said those clots were appearing more frequently in recipients of the AstraZeneca-Oxford vaccine than would be expected in people who had never received the shot.

European regulators had recommended that recipients of the vaccine seek medical assistance for 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. As a result, some emergency room doctors have asked for more central guidance about how to handle what they described as largely unnecessary hospital visits.

German researchers have described specialized blood tests that can be used to diagnose the disorder, and suggested treatment with a blood product called intravenous immune globulin, which is used to treat various immune disorders.

Drugs called anti-coagulants, or blood thinners, can also be administered, but not a commonly used one — heparin — because the vaccine-related condition is very similar to one that occurs, rarely, in people given heparin.

Other vaccines, particularly the one given to children for measles, mumps and rubella, have been linked to temporarily lowered levels of platelets, a blood component essential for clotting.

Lowered platelet levels have been reported in small numbers of patients receiving the Moderna, Pfizer-BioNTech and 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.

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Pfizer Requests Approval to Use Vaccine in Young Teens

Pfizer and BioNTech requested on Friday that the Food and Drug Administration expand the emergency use authorization for their coronavirus vaccine to permit its use in children ages 12 to 15. If approved, it could allow young adolescents to start getting vaccinated before going back to school in the fall.

The companies plan to request similar authorizations from health agencies around the world in the coming days, they said in a joint statement.

“These submissions represent a critical step in Pfizer’s and BioNTech’s ongoing efforts to support governments in broadening global vaccination efforts,” the statement said. Clinical trial results found the vaccine highly effective in that age group, the companies said last month.

The Pfizer-BioNTech vaccine is currently approved for use via emergency authorization in people 16 and older. Granting approval for its use in the younger age group would also speed the country’s efforts to reach herd immunity, which will depend on vaccinating children.

More than 2,000 young adolescents participated in a Phase 3 trial of the vaccine. Among those who received the vaccine, none developed symptomatic coronavirus infections or exhibited serious side effects, the companies said last month. The vaccinated 12- to 15-year-olds also produced higher levels of antibodies, on average, than older adolescents and young adults did.

The trial results have not yet been published in a scientific journal.

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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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To Speed Vaccination, Some Call for Delaying Second Shots

The prospect of a fourth wave of the coronavirus, with new cases climbing sharply in the Upper Midwest, has reignited a debate among vaccine experts over how long to wait between the first and second doses. Extending that period would swiftly increase the number of people with the partial protection of a single shot, but some experts fear it could also give rise to dangerous new variants.

In the United States, two-dose vaccines are spaced three to four weeks apart, matching what was tested in clinical trials. But in Britain, health authorities have delayed doses by up to 12 weeks in order to reach more people more quickly. And in Canada, which has precious few vaccines to go around, a government advisory committee recommended on Wednesday that second doses be delayed even longer, up to four months.

Some health experts think the United States should follow suit. Dr. Ezekiel J. Emanuel, a co-director of the Healthcare Transformation Institute at the University of Pennsylvania, has proposed that for the next few weeks, all U.S. vaccines should go to people receiving their first dose.

“That should be enough to quell the fourth surge, especially in places like Michigan, like Minnesota,” he said in an interview. Dr. Emanuel and his colleagues published the proposal in an op-ed on Thursday in USA Today.

10 days after the first dose, researchers could see that the volunteers were getting sick less often than those who got the placebo.

In the same month, Britain experienced a surge of cases caused by a new, highly transmissible variant called B.1.1.7. Once the British government authorized two vaccines — from Pfizer-BioNTech and AstraZeneca — it decided to fight the variant by delaying the second doses of both formulations by 12 weeks.

said on Jan. 31 on NBC’s “Meet the Press.”

But the government stayed the course, arguing that it would be unwise to veer off into the unknown in the middle of a pandemic. Although the clinical trials did show some early protection from the first dose, no one knew how well that partial protection would last.

“When you’re talking about doing something that may have real harm, you need empirical data to back that,” said Dr. Céline R. Gounder, an infectious-disease specialist at Bellevue Hospital Center and a member of Mr. Biden’s coronavirus advisory board. “I don’t think you can logic your way out of this.”

But in recent weeks, proponents of delaying doses have been able to point to mounting evidence suggesting that a first dose can provide potent protection that lasts for a number of weeks.

The Centers for Disease Control and Prevention reported that two weeks after a single dose of either the Moderna or the Pfizer-BioNTech vaccine, a person’s risk of coronavirus infection dropped by 80 percent. And researchers in Britain have found that first-dose protection is persistent for at least 12 weeks.

Dr. Emanuel argued that Britain’s campaign to get first doses into more people had played a role in the 95 percent drop in cases since their peak in January. “It’s been pretty stunning,” Dr. Emanuel said.

studies that show that a single dose of Moderna or Pfizer-BioNTech does not work as well against certain variants, such as B.1.351, which was first found in South Africa.

“Relying on one dose of Moderna or Pfizer to stop variants like B.1.351 is like using a BB gun to stop a charging rhino,” said John P. Moore, a virologist at Weill Cornell Medicine.

Dr. Moore said he also worried that delaying doses could promote the spread of new variants that can better resist vaccines. As coronaviruses replicate inside the bodies of some vaccinated people, they may acquire mutations that allow them to evade the antibodies generated by the vaccine.

But Dr. Cobey, who studies the evolution of viruses, said she wasn’t worried about delayed doses breeding more variants. “I would put my money on it having the opposite effect,” she said.

Last week, she and her colleagues published a commentary in Nature Reviews Immunology in defense of delaying doses. Getting more people vaccinated — even with moderately less protection — could translate into a bigger brake on the spread of the virus in a community than if fewer people had stronger protection, they said. And that decline wouldn’t just mean more lives were saved. Variants would also have a lower chance of emerging and spreading.

“There are fewer infected people in which variants can arise,” she said.

Dr. Adam S. Lauring, a virologist at the University of Michigan who was not involved in the commentary, said he felt that Dr. Cobey and her colleagues had made a compelling case. “The arguments in that piece really resonate with me,” he said.

Although it seems unlikely that the United States will shift course, its neighbor to the north has embraced a delayed strategy to cope with a booming pandemic and a short supply of vaccines.

Dr. Catherine Hankins, a public health specialist at McGill University in Montreal and a member of Canada’s Covid-19 Immunity Task Force, endorsed that decision, based on the emerging evidence about single doses. And she said she thought that other countries facing even worse shortfalls should consider it as well.

“I will be advocating at the global level that countries take a close look at Canada’s strategy and think seriously about it,” Dr. Haskins said.

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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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AstraZeneca Vaccine Faces 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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U.K. says AstraZeneca alternative should be offered for under 30s, and E.U. finds a ‘possible link’ to rare clots.

Britain said on Wednesday that it would offer alternatives to the AstraZeneca vaccine for adults under 30 as European regulators described a “possible link” with rare blood clots, a setback for the world’s most widely used vaccine and a blow to the more than 100 countries relying on it to save lives amid a global surge in coronavirus cases.

The European regulator, the European Medicines Agency, stopped short of advising that use of the vaccine be curbed in the 27 European Union countries, saying that it was up to the national authorities to decide who should receive which vaccine.

Until the announcement, Britain had never wavered in its use of the vaccine, making it a holdout in Europe even as many countries detected unusual, sometimes fatal, blood clots in some recipients. But evidence has mounted that very small numbers of Britons had also been afflicted, forcing the country to reduce the use in younger people of a vaccine that is the backbone of its world-beating inoculation program.

The concern over the blood clots has threatened the pace of vaccinations far beyond Europe. At least 111 countries of varying income levels have administered doses of AstraZeneca’s shot, making it international aid groups’ most potent weapon in the battle to reduce deaths in the vaccine-starved global south.

finally over rare, but sometimes fatal, blood clots reported in some recipients.

Those concerns led several European countries to first restrict the use of AstraZeneca in older age groups, then suspend it over reports of blood clots, only to roll it out again last month after the European Medicines Agency issued a preliminary opinion that the benefits of the vaccine outweighed the risks.

As doctors reported a higher incidence of serious blood clots in younger people, some countries decided to stop administering the shot to anyone younger than 55.

Europe’s concerns over the vaccine’s side effects are also likely to threaten global inoculation efforts, with much of the developing world depending on the AstraZeneca vaccine to tackle the pandemic. The shot is the cornerstone of Covax, a program designed to make vaccine access more equitable worldwide.

The vaccine appeared to be causing an immune reaction in which antibodies bind to platelets, activating them, German doctors and the European Medicines Agency have said. Those platelets, in turn, were causing the formation of dangerous clots in certain parts of the body, including in veins that drain blood from the brain, leading in some cases to a rare type of stroke.

Why the antibodies develop in these people is not known, doctors have said. Some component of the vaccine, or excessive immune reaction — or both — could be the cause, they said.

No pre-existing conditions are known to make patients more vulnerable to this clotting disorder after a vaccination, European regulators said.

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