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The coronavirus vaccine developed by AstraZeneca and the University of Oxford provided strong protection against Covid-19 in a large clinical trial in the United States, completely preventing the worst outcomes from the disease while causing no serious side effects, according to results announced on Monday.
The findings, announced in a news release from AstraZeneca, may help shore up global confidence in the vaccine, which was shaken this month when more than a dozen countries, mostly in Europe, temporarily suspended the use of the shot over concerns about possible rare side effects.
The trial, involving more than 32,000 participants, was the largest test of its kind for the shot. The vaccine was 79 percent effective overall in preventing symptomatic infections, higher than observed in previous clinical trials. The trial also showed that the vaccine offered strong protection for older people, who had not been as well-represented in earlier studies.
But the fresh data may not make much difference in the United States, where the vaccine is not yet authorized and may not be needed.
If AstraZeneca wins authorization for emergency use in the United States based on the new results, the vaccine is unlikely to become available before May, when federal officials predict that three vaccine manufacturers that already have authorization will be producing enough doses for all the nation’s adults.
AstraZeneca said on Monday that it would continue to analyze the new data and prepare to apply “in the coming weeks” for emergency authorization from the Food and Drug Administration. The vaccine has already been approved in more than 70 countries, but clearance from American regulators, if the company can secure it, would bolster the vaccine’s reputation globally.
The new data has been highly anticipated because of the expectation that the trial, designed to meet F.D.A. standards, would be the best measure of how well the vaccine works. The inexpensive and easy-to-store shot is being counted on as the workhorse of the drive to vaccinate the world, with reliance on it especially high in poor and middle-income countries.
The interim results announced on Monday were based on 141 Covid-19 cases that had turned up in volunteers. Two-thirds of participants were given the vaccine, with doses spaced four weeks apart, and the rest received a saline placebo. Volunteers were recruited from Chile and Peru in addition to the United States.
None of the volunteers who got the vaccine developed severe symptoms or had to be hospitalized, a major selling point for the shot. However, AstraZeneca did not disclose the number of volunteers who got the placebo who developed severe Covid-19 or had to be hospitalized, making it difficult to know how statistically powerful those findings are.
The new results could help the vaccine recover from the safety scare it has faced in Europe. Regulators there initiated a safety review after a small number of people who had recently been inoculated developed blood clots and abnormal bleeding.
Most countries are now restarting use of the shot, with some restrictions and a new warning label, after the European Union’s drug regulator said on Thursday that a review determined that the vaccine was safe. Nonetheless, the speed at which several nations suspended use of the vaccine reflected a skittishness about its safety and effectiveness that contrasts sharply with the confidence that has been shown in other vaccines.
Participants who received the vaccine in the trial had no increased risk of blood clots or related illnesses. And a specific search turned up no cases of cerebral venous sinus thrombosis — blood clots in the brain that can result in dangerous bleeding — that raised some of the most serious concerns in Europe.
The U.S. trial also turned up no cases of serious neurological problems. That arose as a concern last summer after two volunteers in AstraZeneca’s trial in Britain fell ill with neurological problems. Although those cases forced a seven-week halt to the U.S. clinical study, researchers ultimately concluded that the illnesses could not be linked to the vaccine. Still, the delay was a key factor in why AstraZeneca fell so far behind three other vaccine manufacturers that have won emergency authorization in the United States.
The vaccine worked well across ethnicity and age groups, AstraZeneca said. The vaccine was 80 percent efficacious in approximately 6,000 trial enrollees over age 65 — findings likely to quell concerns about insufficient clinical trial data on how well the vaccine works in older people. Some countries briefly limited inoculations with the shot to younger people before reversing course to allow it to be used in all age groups, after data from the vaccine’s rollout in Britain showed the vaccine provided strong protection in older people.
AstraZeneca is poised to quickly release tens of millions of doses for use in the United States if regulators clear the vaccine for emergency use. The company has said it will have up to 50 million doses ready by the end of April. But unlike the three previous manufacturers that won that authorization, AstraZeneca’s doses may not be needed.
Around the time AstraZeneca’s vaccine is poised to get a decision, federal officials expect that the government will have locked in enough supply to cover the nation’s roughly 260 million adults.
Even if the vaccine is not used in the United States, receiving emergency authorization from the F.D.A. — whose rigorous review process is considered the global gold standard — would be an important milestone for AstraZeneca. Some countries that have not yet authorized the vaccine may look to follow the F.D.A.’s lead.
A green light from the agency could also bolster trust in countries where the shot is already authorized — a considerable challenge for the vaccine, which has been perceived by some as less desirable than other shots because of its lower overall efficacy rate in previous clinical trials.
While it is difficult to compare vaccines tested in differently designed clinical trials in different populations at different points in the pandemic, the new results put AstraZeneca’s vaccine in the company of other leading vaccines.
The vaccines from Pfizer and Moderna, which rely on messenger RNA technology and must be stored at very cold temperatures, were both found in large clinical trials to be about 95 percent efficacious overall in preventing symptomatic infections. Johnson & Johnson’s one-shot vaccine had a 72 percent overall efficacy rate in trial participants in the United States.
AstraZeneca’s vaccine can be stored for up to six months when refrigerated. It uses an older approach similar to that of the shot from Johnson & Johnson. A delivery vehicle — a weakened version of a chimpanzee adenovirus — is used to transport coronavirus genes into human cells. That is meant to train the immune system to fight off attacks from the real coronavirus.
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