The largest published study to date of certain post-vaccine side effects found that people were at slightly higher than normal risk of blood clots after receiving an AstraZeneca or Pfizer-BioNTech shot. But the same clotting conditions were substantially more likely to occur — and over longer periods — among people infected with the coronavirus, the study found.Coupled with another study this week from Israel, the data, published on Thursday night in The British Medical Journal, added to the growing evidence that though the coronavirus vaccines are associated with certain rare side effects, those risks are dwarfed by the risks from Covid-19.The study was based on the electronic health records of more than 29 million people in England. It went beyond previous analyses in finding a link not only between very rare clotting conditions and the AstraZeneca vaccine, but also between those conditions and the Pfizer vaccine. Earlier studies had detected increased clotting risks after the AstraZeneca vaccine, but not after the Pfizer shot.In interviews, the new paper’s co-authors said that the numbers of those cases they detected — involving clots blocking a vein that drains blood from the brain — were small enough that further studies were needed. Even the increased risk of those clots was far outweighed by people’s chances of developing them after contracting the virus itself, the study said.“Although there are some risks, clearly the risks of these events are very rare,” said Aziz Sheikh, a co-author of the study and a professor of primary care research at the University of Edinburgh in Scotland. “And the biggest point is that the risks associated with Covid-19 are orders of magnitude higher, really.”The study examined the electronic health records of people given a first dose of a coronavirus vaccine during the first five months of England’s inoculation campaign. Of those 29 million people, nearly 1.8 million people also tested positive for the coronavirus before or after being vaccinated. The study compared the risk of blood clots shortly after vaccination to the risk during other periods, as well as to the weeks after someone developed Covid-19.After a first shot of the AstraZeneca vaccine, people were at slightly increased risk of certain blood clots, as well as a condition characterized by a low level of platelets that can leave them prone to abnormal bleeding. A first shot of the Pfizer vaccine appeared to put people at slightly elevated risk of strokes caused by blockages within a blood vessel.And both vaccines were linked, though at low numbers, to the very rare clots that prevent blood from draining out of the brain.Even so, those risks were far smaller than those linked to developing Covid-19. For example, the authors said, for every 10 million people given a first dose of the AstraZeneca vaccine, about 66 more people than normal would develop clots starting in a vein. But among the same number infected with the virus itself, 12,614 more people than normal would develop those clots.In the United States, 300,000 to 600,000 people a year develop blood clots in their lungs or in leg veins or other parts of the body, according to the C.D.C. With nearly a million people a day now getting vaccinated, some of those clots will occur in those receiving the shots just by coincidence, unrelated to the vaccine..css-1xzcza9{list-style-type:disc;padding-inline-start:1em;}.css-3btd0c{font-family:nyt-franklin,helvetica,arial,sans-serif;font-size:1rem;line-height:1.375rem;color:#333;margin-bottom:0.78125rem;}@media (min-width:740px){.css-3btd0c{font-size:1.0625rem;line-height:1.5rem;margin-bottom:0.9375rem;}}.css-3btd0c strong{font-weight:600;}.css-3btd0c em{font-style:italic;}.css-w739ur{margin:0 auto 5px;font-family:nyt-franklin,helvetica,arial,sans-serif;font-weight:700;font-size:1.125rem;line-height:1.3125rem;color:#121212;}#NYT_BELOW_MAIN_CONTENT_REGION .css-w739ur{font-family:nyt-cheltenham,georgia,’times new roman’,times,serif;font-weight:700;font-size:1.375rem;line-height:1.625rem;}@media (min-width:740px){#NYT_BELOW_MAIN_CONTENT_REGION .css-w739ur{font-size:1.6875rem;line-height:1.875rem;}}@media (min-width:740px){.css-w739ur{font-size:1.25rem;line-height:1.4375rem;}}.css-9s9ecg{margin-bottom:15px;}.css-16ed7iq{width:100%;display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-align-items:center;-webkit-box-align:center;-ms-flex-align:center;align-items:center;-webkit-box-pack:center;-webkit-justify-content:center;-ms-flex-pack:center;justify-content:center;padding:10px 0;background-color:white;}.css-pmm6ed{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-align-items:center;-webkit-box-align:center;-ms-flex-align:center;align-items:center;}.css-pmm6ed > :not(:first-child){margin-left:5px;}.css-5gimkt{font-family:nyt-franklin,helvetica,arial,sans-serif;font-size:0.8125rem;font-weight:700;-webkit-letter-spacing:0.03em;-moz-letter-spacing:0.03em;-ms-letter-spacing:0.03em;letter-spacing:0.03em;text-transform:uppercase;color:#333;}.css-5gimkt:after{content:’Collapse’;}.css-rdoyk0{-webkit-transition:all 0.5s ease;transition:all 0.5s ease;-webkit-transform:rotate(180deg);-ms-transform:rotate(180deg);transform:rotate(180deg);}.css-eb027h{max-height:5000px;-webkit-transition:max-height 0.5s ease;transition:max-height 0.5s ease;}.css-6mllg9{-webkit-transition:all 0.5s ease;transition:all 0.5s ease;position:relative;opacity:0;}.css-6mllg9:before{content:”;background-image:linear-gradient(180deg,transparent,#ffffff);background-image:-webkit-linear-gradient(270deg,rgba(255,255,255,0),#ffffff);height:80px;width:100%;position:absolute;bottom:0px;pointer-events:none;}.css-uf1ume{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-box-pack:justify;-webkit-justify-content:space-between;-ms-flex-pack:justify;justify-content:space-between;}.css-wxi1cx{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-flex-direction:column;-ms-flex-direction:column;flex-direction:column;-webkit-align-self:flex-end;-ms-flex-item-align:end;align-self:flex-end;}.css-12vbvwq{background-color:white;border:1px solid #e2e2e2;width:calc(100% – 40px);max-width:600px;margin:1.5rem auto 1.9rem;padding:15px;box-sizing:border-box;}@media (min-width:740px){.css-12vbvwq{padding:20px;width:100%;}}.css-12vbvwq:focus{outline:1px solid #e2e2e2;}#NYT_BELOW_MAIN_CONTENT_REGION .css-12vbvwq{border:none;padding:10px 0 0;border-top:2px solid #121212;}.css-12vbvwq[data-truncated] .css-rdoyk0{-webkit-transform:rotate(0deg);-ms-transform:rotate(0deg);transform:rotate(0deg);}.css-12vbvwq[data-truncated] .css-eb027h{max-height:300px;overflow:hidden;-webkit-transition:none;transition:none;}.css-12vbvwq[data-truncated] .css-5gimkt:after{content:’See more’;}.css-12vbvwq[data-truncated] .css-6mllg9{opacity:1;}.css-qjk116{margin:0 auto;overflow:hidden;}.css-qjk116 strong{font-weight:700;}.css-qjk116 em{font-style:italic;}.css-qjk116 a{color:#326891;-webkit-text-decoration:underline;text-decoration:underline;text-underline-offset:1px;-webkit-text-decoration-thickness:1px;text-decoration-thickness:1px;-webkit-text-decoration-color:#326891;text-decoration-color:#326891;}.css-qjk116 a:visited{color:#326891;-webkit-text-decoration-color:#326891;text-decoration-color:#326891;}.css-qjk116 a:hover{-webkit-text-decoration:none;text-decoration:none;}Some countries restricted use of the AstraZeneca vaccine after a small number of people in Europe who had received a shot were seriously sickened or killed by a very rare condition characterized by both clotting and abnormal bleeding.Carol Coupland, a co-author on the latest study and a professor of medical statistics affiliated with both the University of Oxford and the University of Nottingham, said that the study could not evaluate that precise condition because the researchers did not have detailed enough readings of patients’ platelet counts.The finding of a slightly elevated clotting risk after the Pfizer vaccine conflicted with other analyses, including the article from Israel published in the New England Journal of Medicine this week. The study from England included far more people, allowing it to home in on rarer categories of clots. It was also designed differently: It studied the same people over time, whereas the Israeli study compared risks in vaccinated and unvaccinated people over the same period.Ben Reis, a co-author of the Israeli study and the director of the predictive medicine group at the Boston Children’s Hospital Computational Health Informatics Program, said that both studies were a testament to the way electronic health records allowed researchers to rapidly pick up on even very rare safety signals and compare the risks to those following a coronavirus infection.“The vaccination decision shouldn’t be made in a vacuum,” he said. “The alternative outcome is the very real risk of being exposed to the virus without vaccination. Those are the two scenarios that should be compared.”

If you haven’t yet mastered the name of the latest coronavirus variant to set nations on edge — B.1.617.2, as evolutionary biologists call it — then fear not: The World Health Organization has proposed a solution.The group said on Monday that it had devised a less technical, and more easily pronounceable, system for naming variants — the mutated versions of the virus that have driven new surges of infections around the world.Variants will be assigned letters of the Greek alphabet in the order in which they are designated potential threats by the W.H.O.B.1.617.2, for example, which has contributed to a deadly surge in India, has been named Delta under the new system. That variant may spread even more quickly than B.1.1.7, the variant discovered in Britain that has contributed to devastating waves of cases globally. (B.1.1.7’s new name is Alpha.)Scientists will keep assigning long strings of letters and numbers to new variants for their own purposes, but they hope that Greek letters will roll off the tongues of nonscientists more easily.There is also a deeper motivation: The letters-and-numbers system was so complicated that many people were referring to variants by the places they were discovered instead (“the Indian variant” for B.1.617.2, for example). Scientists worry that those informal nicknames can be both inaccurate and stigmatizing, punishing countries for investing in the genome sequencing necessary to sound an alarm about new mutations that may well have emerged somewhere else.Whether the Greek letters will stick is another matter. It has been months since experts convened by the W.H.O. began discussing the issue, allowing labels like “the British variant” and “the South African variant” to proliferate in the news media.The experts said they had considered a number of alternatives, like taking syllables from existing words to make new words. But too many of those syllable combinations were already recognizable names of places or businesses, they said.And as it happens, the Greek letters had just been freed up from another task: The World Meteorological Organization said in March that it would no longer use them to name hurricanes.

Amid a deep residue of mistrust, Western cautions on the AstraZeneca and Johnson & Johnson vaccines risk igniting an explosion of damaging anti-vaccine fervor in the global south.Far beyond the United States and Europe, the safety scares engulfing the AstraZeneca and Johnson & Johnson vaccines have jeopardized campaigns to inoculate the world, undercutting faith in two sorely needed shots and threatening to prolong the coronavirus pandemic in countries that can ill afford to be choosy about vaccines.With new infections surging on nearly every continent, signs that the vaccination drive is in peril are emerging, most disconcertingly in Africa.In Malawi, people are asking doctors how to expunge the AstraZeneca vaccine from their bodies.In South Africa, health officials have paused giving the Johnson & Johnson shot, the only one they have, a repeat blow after dropping AstraZeneca from their arsenal in February.And in the Democratic Republic of Congo, not a single person has been vaccinated, despite 1.7 million doses of the AstraZeneca shot languishing in the country since March 2.In those countries and others, Western colonialism and unethical medical practices have left a residue of mistrust in vaccines, which could harden if the perception takes hold that rich countries are dumping second-rate shots on the global south.Already, doctors say, the recent pauses have vindicated 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 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.”Police officers guarding AstraZeneca vaccines after a shipment arrived last month in Lilongwe, the capital of Malawi.Thoko Chikondi/Associated PressAfrican health officials have reacted with fury at the breezy reassurances of American and European lawmakers that people denied the AstraZeneca or Johnson & Johnson shots could be given another vaccine. In much of the world, there are no other vaccines.And even as American health officials stressed that they paused use of the Johnson & Johnson vaccine on Tuesday in “an abundance of caution,” they forced global health officials to begin crafting the difficult case that shots unsafe for the world’s rich were still suited to its poor. “It’s sending vaccine confidence into a crater,” Ayoade Alakija, co-chair of the African Union’s Africa Vaccine Delivery Alliance, said of rich countries’ actions. “It’s irresponsible messaging, and it speaks to the selfishness of the moment that there wouldn’t be more consultation and communication.”What rich countries call caution, poorer nations will experience as a devastating gamble with the survival of their citizens against Covid-19. “Out of an abundance of caution, let us not destroy vaccine confidence in places that only have access to one type of vaccine,” Dr. Alakija said.The AstraZeneca and Johnson & Johnson vaccines, cheaper and easier to store than the alternatives, are mainstays of global inoculations. AstraZeneca’s shot is being used in at least 118 countries. Lately, amid shortages of that vaccine, some regions have pivoted to Johnson & Johnson’s: Two weeks ago, the African Union acquired 400 million doses.Together, the two vaccines account for a third of the portfolio of Covax, the international effort to procure and distribute vaccines.But it is becoming more apparent by the day that those shots, built on a relatively rugged vaccine platform, are becoming afterthoughts in wealthy nations. After canceling Johnson & Johnson appointments over concerns about rare blood clots, American states offered people the pricier Pfizer or Moderna vaccines instead. The European Union said on Wednesday that it had acquired another 50 million doses of the Pfizer vaccine, allowing it to curb use of AstraZeneca’s vaccine and phase it out altogether next year.Those decisions, intended for domestic audiences, have nevertheless reverberated in countries where variants are spreading, physical distancing is a luxury and there is no choice of shots. Health officials fear that any setbacks in vaccinations could sow the seeds of the next calamitous outbreak, one that deluges hospitals and exports new mutations around the world. In those places, doctors said, the math is obvious: Many more people will die without the AstraZeneca and Johnson & Johnson vaccines than with them.Amid the clotting concerns, the World Health Organization and African Union have not wavered in recommending the use of the AstraZeneca and Johnson & Johnson’s vaccines. In Britain, AstraZeneca’s vaccine remains the backbone of the country’s speedy inoculation campaign, despite people under 30 being offered alternatives. Congo, after spurning the AstraZeneca shot in light of unease in Europe, said on Tuesday that it would launch the much-delayed inoculations next week.A vaccination center inside Salisbury Cathedral, in Salisbury, England, in January. Britain has forged ahead with the AstraZeneca vaccine, though restricting its use to people aged 30 and above.Andrew Testa for The New York TimesAnd in Dakar, the capital of Senegal, people continued to line up on wooden benches on Wednesday for the AstraZeneca shot as they watched their children run through the corridors of a medical center.“We don’t have a choice,” said Alioune Badara Diagne, 34, who lives in the city’s lively Ouakam neighborhood. Despite talk of vaccination pauses in wealthy nations and rumors of vaccine makers using Africans as “guinea pigs,” he said, Westerners themselves were continuing to be injected. He added, “The vaccine is our only hope.”But in much of the world, the American regulators who endorsed Tuesday’s pause on Johnson & Johnson vaccinations act as sort of surrogate decision makers on drugs and vaccines, giving their hesitation extra weight in African nations.“I became even more skeptical when I heard that the United States suspended Johnson & Johnson,” said Lawmond Lawse Nwehla, 32, an engineer in Dakar. “They said it was effective and then they stopped it. So I wonder why.”In immediately pausing the use of Johnson & Johnson’s shot, American regulators reacted more aggressively than did their British counterparts, who backed the AstraZeneca vaccine even as they investigated clotting cases..css-1xzcza9{list-style-type:disc;padding-inline-start:1em;}.css-rqynmc{font-family:nyt-franklin,helvetica,arial,sans-serif;font-size:0.9375rem;line-height:1.25rem;color:#333;margin-bottom:0.78125rem;}@media (min-width:740px){.css-rqynmc{font-size:1.0625rem;line-height:1.5rem;margin-bottom:0.9375rem;}}.css-rqynmc strong{font-weight:600;}.css-rqynmc em{font-style:italic;}.css-yoay6m{margin:0 auto 5px;font-family:nyt-franklin,helvetica,arial,sans-serif;font-weight:700;font-size:1.125rem;line-height:1.3125rem;color:#121212;}@media (min-width:740px){.css-yoay6m{font-size:1.25rem;line-height:1.4375rem;}}.css-1dg6kl4{margin-top:5px;margin-bottom:15px;}.css-16ed7iq{width:100%;display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-align-items:center;-webkit-box-align:center;-ms-flex-align:center;align-items:center;-webkit-box-pack:center;-webkit-justify-content:center;-ms-flex-pack:center;justify-content:center;padding:10px 0;background-color:white;}.css-pmm6ed{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-align-items:center;-webkit-box-align:center;-ms-flex-align:center;align-items:center;}.css-pmm6ed > :not(:first-child){margin-left:5px;}.css-5gimkt{font-family:nyt-franklin,helvetica,arial,sans-serif;font-size:0.8125rem;font-weight:700;-webkit-letter-spacing:0.03em;-moz-letter-spacing:0.03em;-ms-letter-spacing:0.03em;letter-spacing:0.03em;text-transform:uppercase;color:#333;}.css-5gimkt:after{content:’Collapse’;}.css-rdoyk0{-webkit-transition:all 0.5s ease;transition:all 0.5s ease;-webkit-transform:rotate(180deg);-ms-transform:rotate(180deg);transform:rotate(180deg);}.css-eb027h{max-height:5000px;-webkit-transition:max-height 0.5s ease;transition:max-height 0.5s ease;}.css-6mllg9{-webkit-transition:all 0.5s ease;transition:all 0.5s ease;position:relative;opacity:0;}.css-6mllg9:before{content:”;background-image:linear-gradient(180deg,transparent,#ffffff);background-image:-webkit-linear-gradient(270deg,rgba(255,255,255,0),#ffffff);height:80px;width:100%;position:absolute;bottom:0px;pointer-events:none;}#masthead-bar-one{display:none;}#masthead-bar-one{display:none;}.css-1pd7fgo{background-color:white;border:1px solid #e2e2e2;width:calc(100% – 40px);max-width:600px;margin:1.5rem auto 1.9rem;padding:15px;box-sizing:border-box;}@media (min-width:740px){.css-1pd7fgo{padding:20px;width:100%;}}.css-1pd7fgo:focus{outline:1px solid #e2e2e2;}#NYT_BELOW_MAIN_CONTENT_REGION .css-1pd7fgo{border:none;padding:20px 0 0;border-top:1px solid #121212;}.css-1pd7fgo[data-truncated] .css-rdoyk0{-webkit-transform:rotate(0deg);-ms-transform:rotate(0deg);transform:rotate(0deg);}.css-1pd7fgo[data-truncated] .css-eb027h{max-height:300px;overflow:hidden;-webkit-transition:none;transition:none;}.css-1pd7fgo[data-truncated] .css-5gimkt:after{content:’See more’;}.css-1pd7fgo[data-truncated] .css-6mllg9{opacity:1;}.css-1rh1sk1{margin:0 auto;overflow:hidden;}.css-1rh1sk1 strong{font-weight:700;}.css-1rh1sk1 em{font-style:italic;}.css-1rh1sk1 a{color:#326891;-webkit-text-decoration:underline;text-decoration:underline;text-underline-offset:1px;-webkit-text-decoration-thickness:1px;text-decoration-thickness:1px;-webkit-text-decoration-color:#ccd9e3;text-decoration-color:#ccd9e3;}.css-1rh1sk1 a:visited{color:#333;-webkit-text-decoration-color:#ccc;text-decoration-color:#ccc;}.css-1rh1sk1 a:hover{-webkit-text-decoration:none;text-decoration:none;}The costs of the American approach were already evident in Europe, where many nations stopped and then restarted AstraZeneca vaccinations, only to find that it had become a pariah. Most people in France, Germany and Spain distrusted the vaccine.“Once you take the cork out of the bottle, I’m not sure you can get it back in particularly easily,” said Anthony Cox, a vaccines safety expert at the University of Birmingham in England. South Africa immediately copied the American pause on Johnson & Johnson vaccinations, infuriating doctors who are still clamoring for shots, especially in remote parts of the country. In February, health officials dropped the AstraZeneca vaccine over its limited efficacy against a dangerous variant there.To date, only half of 1 percent of the population is vaccinated, and a mere 10,000 shots are being given each day. At that rate, it could take weeks, if not longer, for a single rare blood clotting case to emerge, said Jeremy Nel, an infectious disease doctor in Johannesburg. He was dismayed by the decision to pause shots, given the risk to vaccine confidence in a country where two-fifths of the people say they have no intention of being vaccinated.“The slower you go, that failure is measured in death,” Dr. Nel said. “Even if you delay for a week, there is a non-trivial chance that will cost lives.”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.

The company defended its claims of 79 percent efficacy for its coronavirus shot and said it would release more up-to-date trial results within 48 hours.LONDON — The announcement this week that the AstraZeneca shot, the workhorse of global vaccine rollouts, had achieved nearly 80 percent efficacy in a gold-standard American trial was met with relief by the many countries relying on it.“When you get the call, get the jab,” the British health secretary, Matt Hancock, urged, part of a campaign by European lawmakers to calm people’s nerves after a recent safety scare with the shot.But by Tuesday, that campaign had, once again, been thrown off course, at least for the moment. For AstraZeneca, it was seemingly another episode of public relations whiplash, part of a series of recent miscues and communication blunders by the company that scientists said had undercut the effort to sell one of the most potent and indispensable vaccines against the coronavirus.In a highly unusual move, American health officials said on Tuesday that the company’s account of its U.S. trial findings had not been entirely accurate, suggesting that AstraZeneca had used only the most favorable data to generate apparently spectacular efficacy results.Those comments created new friction between AstraZeneca and American officials even as the company vies for a coveted authorization from the Food and Drug Administration. But more urgently, they threw a wrench into the efforts of elected leaders around the world to rebuild trust in a shot whose low price and easy storage requirements have made it the backbone of many countries’ campaigns to end the pandemic.“It’s eroding confidence,” said Simon Clarke, an associate professor in cellular microbiology at the University of Reading. “When you pump things up, and then people not unreasonably question it, then that erodes confidence.”A vaccination center in Belgrade, Serbia, this month. Despite troubling news reports about the AstraZeneca vaccine, European and global regulators have deemed the shots safe and effective.Laura Boushnak for The New York TimesFaith in the vaccine had already plunged across Europe after recent reports that a very small number of recipients had developed unusual blood clots. In France, Germany, Italy and Spain, more people now believe that the vaccine is unsafe than that it is safe, polling has shown, a blow to a shot that remains the continent’s best hope for saving people’s lives during a mounting surge of new infections.Despite the drumbeat of troubling news reports about the vaccine, European and global regulators have deemed it safe and effective. More than 11 million doses have been administered in Britain alone, almost all of them without serious side effects, driving down hospitalizations and helping the country to emerge from a dreadful wintertime wave of infections.Nevertheless, AstraZeneca’s U.S. trial was hotly anticipated. The largest of its kind for the shot, it had been expected to provide the cleanest, most complete picture of the vaccine’s efficacy. American officials saw it as an incontrovertible test of the vaccine’s performance.And health officials around the world were looking to it as a crucial guide to their own rollouts: It would supply crucial data on older people, who had not been as well represented in earlier trials, and a more precise read on the vaccine’s overall efficacy, which had appeared from earlier trials to be lower than that of other leading shots.As soon as AstraZeneca announced its results on Monday, saying that the vaccine had 79 percent efficacy in preventing symptomatic Covid-19, lawmakers began citing it as part of their fledgling efforts to shore up public confidence in the vaccine.Prime Minister Boris Johnson of Britain received an AstraZeneca shot on Friday. “I literally did not feel a thing,” he later said. “I cannot recommend it too highly.”Pool photo by Frank AugsteinBy Tuesday, scientists said, it seemed as though AstraZeneca had punched a hole in those efforts. Rather than sewing up questions about the shot, it had called to mind communications problems that have dogged the company since last year, delaying the regulatory process in some regions and creating hesitation among some recipients.Scientists said that such a public dust-up between the American medical experts overseeing a trial and the company sponsoring it was extremely unusual.“It is usually done in private,” Stephen Evans, a professor of pharmacoepidemiology at the London School of Hygiene and Tropical Medicine, said of disagreements between a data and safety monitoring board — the experts overseeing the trial — and a vaccine maker. “So this is unprecedented, in my opinion.”In its first public comments, AstraZeneca said that the results published on Monday reflected its U.S. trial data up to Feb. 17. It said that its preliminary assessment of more complete trial data showed that “the results were consistent with the interim analysis,” but said that it would share more up-to-date efficacy results within 48 hours.Scientists said that the problem could yet turn out to be a technical matter that did not change their assessment of the vaccine. American officials did not suggest that any safety issues had been withheld, a subject of intense interest in the wake of the concerns in Europe.AstraZeneca said that the results published on Monday reflected its U.S. trial data up to Feb. 17.  American officials did not suggest that any safety issues had been withheld.Alessandro Grassani for The New York TimesNevertheless, it quickly took the wind out of the sails of European lawmakers’ public campaign to restore confidence in the shot. In recent days, a number of political leaders, including Prime Minister Boris Johnson of Britain and Prime Minister Jean Castex of France, have received the vaccine themselves in a bid to show people that it was safe.“I literally did not feel a thing,” Mr. Johnson told reporters. “I cannot recommend it too highly.”The stumble on Tuesday was the latest in a series of miscues that have made for a prickly relationship between AstraZeneca and American and European regulators — and, scientists said, created unnecessary public confusion about a vaccine that appears to be highly effective.In early September, the company quietly halted its global trials after a participant in Britain fell ill. But American regulators did not find out until the story broke publicly. Subsequently, the company’s slowness to provide the F.D.A. with evidence that its vaccine was not linked to any illnesses kept it grounded for nearly seven weeks. AstraZeneca has said it shared data in a timely manner.By late November, the company was again riding high: It released results from early clinical trials, including in Britain, showing that the vaccine was up to 90 percent effective.But those results, too, were quickly clouded by uncertainty. AstraZeneca later acknowledged that there had initially been confusion over the vaccine dosage received by some study participants, making it more difficult to interpret the findings.Britain, which has long championed the homegrown vaccine, authorized the shot in late December, relying on the earlier clinical trial results. The European Union’s medicine regulator did the same, but a month later.E.U. officials said that the delay had partly resulted from a back-and-forth between the regulators and AstraZeneca over the quality of the data.Waiting outside a vaccination center in Munich, Germany, on Sunday. Europe has seen a mounting surge of new infections recently.Laetitia Vancon for The New York TimesAnd even after the vaccine was authorized, a number of European countries initially restricted it to younger people, citing a lack of sufficient data about its efficacy in older people. That problem was supposed to be resolved by the American trial, in which older people were better represented.Neither European nor British regulators gave any indication on Tuesday that the problems with AstraZeneca’s American data would have any impact on rollouts there. Those agencies relied on a separate set of data from non-American trials to authorize the vaccine.“We are in contact with the company regarding this further information,” the European Medicines Agency said in a statement on Tuesday, “and E.M.A. will assess the data concerned as soon as the company submits it to us.”Matina Stevis-Gridneff contributed reporting from Brussels.