Pfizer said the interim analysis was conducted after 94 participants in the trial developed COVID-19, examining how many of them received the vaccine versus a placebo.

The company did not break down exactly how many of those who fell ill received the vaccine. Still, over 90% effectiveness implies that no more than 8 of the 94 people who caught COVID-19 had been given the vaccine, which was administered in two shots about three weeks apart.

The efficacy rate is well above the 50% effectiveness required by the U.S. Food and Drug Administration for a coronavirus vaccine.

To confirm its efficacy rate, Pfizer said it will continue the trial until there are 164 COVID-19 cases among participants. Given the recent spike in U.S. infection rates, that number could be reached by early December, Gruber said.