The US Food and Drug Administration (FDA) issued a briefing document Tuesday saying the Pfizer-BioNTech Covid-19 vaccine is safe and effective, raising expectations the regulator is poised to grant emergency approval.
An independent committee advising the FDA will meet Thursday on the matter, after Britain became the first country to approve the vaccine last week.
Data from 38,000 US trial participants revealed "suggest a favorable safety profile, with no specific safety concerns identified that would preclude issuance of an EUA (emergency use authorization)," the FDA document said.
It added that the vaccine's efficacy in preventing Covid-19 was 95 percent, worked uniformly across age groups, genders, and racial groups, as well as people with underlying conditions who are at high risk.
The FDA said more research was needed to confirm evidence that the vaccine prevents even the worst cases of Covid-19, that it offers protection after the first of two doses and that it works for people previously been infected.
The companies have previously released some of their data in press statements but the FDA has access to a much more detail.
In a larger group of 43,000 volunteers which included additional adults and adolescents who signed up later, the most common side effects were injection site reactions (84 percent), fatigue (63 percent), headache (55 percent), muscle pain (38 percent), chills (31 percent), joint pain (23.6 percent), fever (14 percent).
Reactions graded as "severe" were more common after the second dose and less frequent in participants aged 55 and older.
There were some adverse events classed as "serious" -- the highest grade -- but there was no meaningful imbalance in the frequency these occurred in the vaccine and placebo arms.
There were four cases of Bell's palsy -- a non-serious form of facial paralysis that generally resolves -- in the vaccine group and none in the placebo group, but the document said this was in line with the frequency seen in the general population.
The advisory group will also consider what additional studies the manufacturers will need to undertake after issuance of an EUA.
Notably, the FDA is not in favor of immediately offering the vaccine to people in the placebo group, which is currently a heated area of discussion among the scientific community.