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

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officials said early Tuesday that results from a U.S. trial of AstraZeneca’s Covid-19 vaccine may have relied on “outdated information” that “may have provided an incomplete view of the efficacy data,” casting doubt on an announcement on Monday that had been seen as good news for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

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

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

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

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

In recent days, the monitoring board’s analysis was delayed several times because the board had to ask for revised reports from those handling trial data on behalf of the company, according to a person familiar with the matter who was not authorized to discuss it publicly.

Companies sponsoring drug or vaccine trials typically wait for the monitoring board to run analyses and conclude that the study has yielded an answer before they announce trial results. During the trial, an unblinded statistical analysis group that is walled off from the company acts as an intermediary to handle data requests and many other interactions with the monitoring board.

Company executives are blinded to the results of the study until the monitoring board reports their study data back to them. The monitoring board ultimately conveyed the results of the study to AstraZeneca in a meeting over the weekend, leading to the company’s announcement Monday morning.

An AstraZeneca spokeswoman, whom the company declined to name, said last week that it was “completely incorrect” that the trial data had formatting problems or had not been submitted to the monitoring board in a clean fashion.

“As is often the case,” the spokeswoman said, monitoring boards “can request new or clarifying analyses of data from the trial. This would enable them to ensure the robustness of their determinations.”

Dr. Eric Topol, a clinical trials expert at Scripps Research in San Diego, said it was “highly irregular” to see such a public display of friction between a monitoring board and a study sponsor, which are typically in close concordance.

“I’ve never seen anything like this,” he said in an interview after the institute’s statement was released. “It’s so, so troubling.”

AstraZeneca’s relationship with the U.S. authorities has been fraught since last year, when senior health officials believed the company was not being forthright about the design of its clinical trials, its results and safety issues. That skepticism carried over to last week, when senior officials at a number of federal health agencies grew suspicious about why AstraZeneca had not announced data from its U.S. study.

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

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

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

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

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

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

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

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

Last summer, at least some top F.D.A. officials learned only from news reports that AstraZeneca had paused its Phase 2/3 vaccine trial in Britain after a participant developed neurological symptoms. Then in September, after another participant in the British study fell ill with similar symptoms, AstraZeneca halted its trials globally but failed to promptly notify the U.S. authorities.

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

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