The newly released documents, which include the F.D.A.’s first technical analysis of the company’s 45,000-person clinical trial, presented evidence that the vaccine was safe, with noticeably milder side effects than the Pfizer and Moderna vaccines and without any reports of severe allergic reactions like anaphylaxis.
The vaccine’s protection was consistent across Black, Hispanic and white volunteers, and also across different ages. The trial estimated a lower efficacy, of 42.3 percent, for people over 60 who had risk factors like heart disease or diabetes, a figure that came with a large amount of statistical uncertainty, the F.D.A. noted.
Dr. James Burke, an expert on trial design at the University of Michigan School of Medicine, cautioned that the results found in small subgroups can turn out to be the result of chance. “We’re wrong more than we’re right,” he said. “So we should always tread very cautiously.”
He noted that the trial only recorded 41 cases of Covid-19 in 6,667 people over 60 with comorbidities. “Common sense makes it pretty clear that we can’t make very robust estimates with such a small number of outcomes,” Dr. Burke said.
Preliminary data suggests that the vaccine’s protective effects grow in the weeks after vaccination. After 42 days, for example, only one vaccinated person got Covid-19, whereas 13 people in the placebo group did, which translates to a 92.4 percent efficacy rate. It’s not clear how long the vaccine’s protection will last before it wanes, an uncertainty that hovers over all the coronavirus vaccines, since they have only gone into testing in recent months.
Although several vaccines can protect people from getting sick with Covid-19, it is unclear whether the shots can also prevent people from getting infected and passing the virus to others, leading to a debate about how quickly society can return to normal after inoculations begin.
Moderna’s trial found some hints that vaccinated people were less likely to develop an infection without symptoms. And AstraZeneca found that its vaccine reduced asymptomatic infections by about half.
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