According to the United States Centers for Disease Control and Prevention (CDC), the US currently has the safest vaccine supply in national history. The Food and Drug Administration (FDA) requires extensive clinical trials before it approves any vaccine for use. These include three phases of testing on volunteers, who report on safety and efficacy as well as common side effects.
If the vaccine proves safe and effective, and if its benefits outweigh the risk of side effects, the FDA licenses its production. This authorizes the manufacturing of the vaccine in batches, each of which must undergo testing before the FDA authorizes its release.
After the vaccine's release, the FDA continues to require post-licensure studies to ensure the continuing safety of each vaccine and to examine its use in potentially under-represented patient populations. The FDA and CDC also monitor the vaccine to identify any potential adverse events.
The Advisory Committee on Immunization Practices (ACIP) is responsible for recommending those vaccines deemed to be safe and beneficial to children. Testing continues indefinitely on all vaccines added to the US Recommended Immunization Schedule, and recommendations may change if testing reveals a new side effect or adverse event.