How do we know a vaccine is safe?

The FDA reviews all vaccines for safety before allowing them onto the market. If problems are discovered with a vaccine after its release, the FDA can recommend a pause in its use, as it did with the Johnson & Johnson vaccine after reports of rare but significant blood clots in several people who had taken the vaccine. After reviewing the data, the FDA decided to continue allowing its use with a warning about the rare clotting events on the label.

(Updated 4/26/21)