The companies said that of the 170 cases of Covid-19, 162 were in the placebo group, and eight were in the vaccine group. Of the 10 cases of severe Covid-19, nine have received a placebo.

Pfizer and BioNTech said the vaccine efficacy is consistent across age, race and ethnicity. Fatigue was the most serious adverse event, with 3.7 percent of volunteers reporting fatigue after taking the second dose. Two percent of the volunteers reported a headache after the second dose. The companies said that older adults reported fewer and less severe side effects.

“It’s really amazing,” said Akiko Iwasaki, an immunologist at Yale University. She said results in people over the age of 65 were the most promising. “We know from the influenza vaccine that it is very difficult to achieve protection in this age group with vaccines,” she said, so the effectiveness of 94 percent in that group is “really impressive.”

Federal health officials said the first doses of vaccines will likely go to groups such as healthcare workers who are at high risk of exposure, as well as to people most at risk of contracting the disease, such as the elderly.

Dr. Saad B. Omar, director of the Yale Institute for Global Health, said the results showing that vaccines protect people from severe disease were also good news, because with such limited availability at first, the first goal would not be to stop transmission. This disease, but to prevent people from becoming very sick. He said, “This is very reassuring.”

And he called on Pfizer to quickly release a more detailed analysis of the data – unlike its initial news release – so that scientists can evaluate the results more comprehensively. The companies said they plan to present the results for review in a scientific journal, a process that could take weeks or months. “These are not normal times, this is a pandemic, and a major policy is being developed,” said Dr. Omar. “It is always better to have a policy on scientific data, not press releases.”

In addition to the results of its clinical trial, Pfizer said it is ready to provide two months of safety data to the FDA recommended by the agency, as well as detailed manufacturing records showing that the company can consistently produce high-quality batches of its products. . The Food and Drug Administration (FDA) will review the data and ask an external panel of vaccine experts to consider the request, a process that could take weeks.