All of the vaccines passed rigorous safety reviews by the FDA.

The agency authorized the Pfizer-BioNTech vaccine for people ages 16 and over after reviewing data from 44,000 participants in a randomized clinical trial. A 53-page analysis by the agency found that some people who received injections had unpleasant but tolerable side effects, including fatigue, headaches, muscle pain, joint pain, chills and fever. Reviewers said the two months of follow-up on 38,000 of those participants provide evidence of “a favorable safety profile, with no specific safety concerns identified that would preclude issuance of an [emergency authorization].”

The FDA cleared the Moderna vaccine for people ages 18 and over, based on data from a clinical trial with 30,000 participants. The agency found side effects were similar to those with the Pfizer-BioNTech shot.

The Johnson & Johnson vaccine was authorized for people ages 18 and over, based on data from a trial with 44,000 participants. Side effects from the vaccine were less common than those for Moderna and Pfizer-BioNTech and included pain at the injection site, fatigue and headaches.

Some experts have expressed qualms about using emergency-use authorizations for coronavirus vaccines that would be given to hundreds of millions of people, but those criticisms have become muted as the pandemic has raged, killing thousands of Americans a week.

Peter Marks, director of the FDA center that oversees vaccines, has pledged to use an emergency standard roughly equivalent to what is needed for a full licensure. Even so, the available safety data — two months of follow-up on half the trials’ participants after their second shots — is shorter than in traditional trials. And some questions, such as the duration of protection, cannot be answered now. That data will be collected as the trials continue.

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