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. Trust in the vaccine has tumbled in Germany, France, Italy, Spain and, to a lesser degree, Britain, according to polls.

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.

Michael Head, a senior research fellow in global health at the University of Southampton in Britain, said that the results could allay concerns not only in Europe but also globally. He said that he had received messages in recent days from colleagues in Ghana, fretting about how to explain the safety scare to people who had been celebrating the vaccine’s arrival only weeks earlier.

“Because of the slight battering the AstraZeneca vaccine has taken over the last few weeks — and particularly in the last week or two in Europe — new data showing it is safe and effective is, if you’ll excuse the phrase, a good shot in the arm,” he said. “The publication of these results is actually quite well-timed, given possible hesitancy around the vaccine.”

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.

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.

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AstraZeneca’s Covid-19 Vaccine Is Found to Be 79% Effective in U.S. Study

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