Pfizer says an early peek at its vaccine data suggests the shots maybe 90 per cent effective at preventing COVID-19, indicating the company is on track later this month to file an emergency use application with U.S. regulators.

Monday's announcement doesn't mean a vaccine is imminent: This interim analysis, from an independent data monitoring board, looked at 94 infections recorded so far in a study that has enrolled nearly 44,000 people in the US and five other countries.

Pfizer Inc did not provide any more details about those cases and cautioned the initial protection rate might change by the time the study ends.

Even revealing such early data is highly unusual. "We're in a position potentially to be able to offer some hope," Dr Bill Gruber, Pfizer's senior vice president of clinical development, told The Associated Press. "We're very encouraged." Authorities have stressed it's unlikely any vaccine will arrive much before the end of the year, and limited initial supplies will be rationed.

The shots made by Pfizer and its German partner BioNTech are among 10 possible vaccine candidates in late-stage testing around the world — four of them so far in huge studies in the US.

Another US company, Moderna Inc, also has said it hopes to be able to file an application with the Food and Drug Administration later this month.

Volunteers in the final-stage studies, and the researchers, don't know who received the real vaccine or a dummy shot.

But a week after their second required dose, Pfizer's study began counting the number who developed COVID-19 symptoms and were confirmed to have the Coronavirus.

Because the study hasn't ended, Gruber couldn't say how many in each group had infections.

Doing the math, that would mean almost all the infections counted so far had to have occurred in people who got the dummy shots.