AstraZeneca on Monday released preliminary clinical trial data on its Covid-19 vaccine candidate that showed the inoculation is safe and effective at preventing asymptomatic and severe cases of the disease, setting the stage for the company to seek FDA's authorization for the vaccine.

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The new data comes after numerous countries last week suspended the use of the vaccine, which AstraZeneca developed with the University of Oxford, due to a small number of reports of blood clots among those who had received the inoculation.

However, the European Union's drug regulator, called the European Medicines Agency (EMA), and the World Health Organization (WHO) have said there is little reason to believe the vaccine caused the blood clots, which can occur by chance in otherwise healthy populations. Most countries have since resumed administering the vaccine.

Editor's note: The National Institute of Allergy and Infectious Diseases in a statement early on March 23 said AstraZeneca may have released "outdated information" on its Covid-19 vaccine that "may have provided an incomplete view" of the vaccine's effectiveness. AstraZeneca responded that the numbers "were based on a pre-specified interim analysis" and that it would "immediately engage with the independent Data Safety Monitoring Board to share our primary analysis with the most up to date efficacy data."

Data shows AstraZeneca's Covid-19 is safe and effective

The new data is from a large-scale clinical trial. For the trial, researchers recruited and enrolled more than 32,000 participants from Chile, Peru, and the United States to test the vaccine candidate's effectiveness and safety.

Among the trial's participants, approximately 79% were white, 22% were Hispanic, 8% were Black, 4% were Native American, and 4% were Asian. Roughly 20% of participants were ages 65 and older, and about 60% had comorbidities—including cardiac disease, diabetes, and severe obesity—associated with severe cases of Covid-19. The participants received either two shots of the vaccine candidate or two saline shots administered four weeks apart.

The vaccine also was 100% effective in preventing severe cases of Covid-19 and hospitalizations—with no severe cases or hospitalizations occurring among the participants who had received the vaccine, according to the release. However, it's difficult determine whether those findings represent a statistically significant reduction in risk compared to a placebo, because AstraZeneca did not release data on the number of placebo recipients who developed severe cases of Covid-19 or had to be hospitalized, the Times reports.

In terms of safety, the data showed no increased risk of blood clots or related events among the 21,583 participants who received at least one dose of the vaccine. The data also showed no cases of serious neurological problems. And when researchers reviewed their data for evidence of blood clots in the brain—which have been reported among vaccine recipients in Europe—they found no cases, according to the release.

AstraZeneca in the release said it "will continue to analyze the data and prepare for the primary analysis to be submitted to [FDA] for [an EUA] in the coming weeks."

According to experts, any authorization in the United States likely wouldn't happen until May, when federal officials project the United States will have enough vaccine doses for all American adults from the three manufacturers who already offer authorized vaccines. As a result, AstraZeneca's vaccine is expected to have a minimal effect on America's Covid-19 vaccine rollout, the Times reports.

Experts say the data is reassuring but want more details

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Many experts said the new data is reassuring amid the concerns about the vaccine's safety.

In the release, Ann Falsey, a professor of medicine at the University of Rochester School of Medicine and co-lead principal investigator for the clinical trial, said, "This analysis validates the AstraZeneca Covid-19 vaccine as a much-needed additional vaccination option, offering confidence that adults of all ages can benefit from protection against the virus."

Similarly, Paul Hunter, a professor of medicine at the University of East Anglia, said the preliminary results are encouraging, but more information is needed to confirm the vaccine's effectiveness at preventing severe cases of Covid-19 and hospitalizations.

"It would be good to know how many severe cases occurred in the control group and so what the confidence intervals are for this 100% figure," Hunter said. "But this should add confidence that the vaccine is doing what it is most needed for."

Stephen Evans, of the London School of Hygiene & Tropical Medicine, said, "The benefits of these results will mainly be for the rest of the world, where confidence in the … vaccine has been eroded, largely by political and media comment" (Robbins/Mueller, New York Times, 3/22; Strasburg, Wall Street Journal, 3/22; Cheng/Neergaard, Associated Press, 3/22; Herper, STAT News, 3/22; Joseph, STAT News, 3/20; New York Times, 3/15; Anderson, Becker's Hospital Review, 3/19).