The experimental vaccine had already shown promise at an early analysis announced last week, but the trial sped to completion faster than anticipated due to the spike in coronavirus cases.
In the trial, half the nearly 44,000 participants received the experimental vaccine and half received a placebo. As those people went about their normal lives, they were exposed to the virus in the community, and physicians tracked all cases with symptoms to see if the vaccine had a protective effect.
The data have not yet been published or peer reviewed, but will be closely scrutinized by the Food and Drug Administration and an independent advisory committee that makes recommendations to the agency.
Among 170 cases of covid-19 in the trial, 162 were in the placebo group and eight were in the vaccine group. There were 10 cases of severe disease in the trial, nine of which were in the placebo group and one in the vaccine group.
Among people older than 65, a group at high risk of severe illness, the vaccine was 94 percent effective.
“We continue to move at the speed of science to compile all the data collected thus far and share with regulators around the world,” Pfizer chief executive Albert Bourla said in a statement.
The news comes days after Moderna, a biotechnology company, announced its vaccine was nearly 95 percent effective at an early analysis.
U.S. government officials anticipate having 40 million doses of both vaccines by the end of the year, enough to vaccinate 20 million people.