A so-called interim analysis from the company examined the first 94 confirmed cases of Covid-19 among the more than 43,000 volunteers who received either two doses of the vaccine or a placebo.
"With today’s news, we are a significant step closer to providing people around the world with a much-needed breakthrough to help bring an end to this global health crisis,” Pfizer CEO Albert Bourla said in a statement.
“We look forward to sharing additional efficacy and safety data generated from thousands of participants in the coming weeks.”
The analysis found fewer than 10% of infections were in participants who had been given the vaccine. More than 90% of the cases were in people who had been given a placebo.
Pfizer said that the vaccine provided protection seven days after the second vaccine dose and 28 days after the initial dose.
The company's final goal is to reach 164 confirmed cases of coronavirus infection.
In a press release, the pharmaceutical giant said it plans to seek emergency use authorization from the US Food and Drug Administration (FDA) soon after volunteers have been monitored for two months after getting their second dose of vaccine, as requested by the FDA.
Pfizer said it anticipated reaching that point by the third week of November. Its vaccine trial is conducted in partnership with German company BioNTech.
The Phase 3 vaccine trial has enrolled 43,538 participants since July 27. As of Sunday, 38,955 of the volunteers have received a second dose of the vaccine. The company says 42% of international trial sites and 30% of US trial sites involve volunteers of racially and ethnically diverse backgrounds.
Pfizer's study will also evaluate whether the vaccine protects people against severe Covid-19 disease and whether the vaccine can provide long-term protection against Covid-19 disease, even in patients who have been infected before.
The US FDA has said it would expect at least 50% efficacy from any coronavirus vaccine.
BioNTech's shares rose sharply after the news.