Vaccination is one of the most highly effective public health interventions, responsible for saving millions of lives each year. In the US, authorized or approved preventive vaccines must be manufactured with high quality, and the effectiveness and favorable safety profile of vaccines must be demonstrated. Their safety over time is also closely and continuously monitored through multiple overlapping passive and active safety surveillance systems, including the Vaccine Adverse Event Reporting System, the Vaccine Safety Datalink, and the BEST Sentinel Initiative.1 Despite the care taken in the development and deployment of vaccines and their clear and compelling benefit of saving individual...