The Centers for Disease Control and Prevention and the Food and Drug Administration have multiple surveillance systems for ensuring vaccine safety. If a health issue occurs after vaccination, people are encouraged to submit information to one of these systems, the Vaccine Adverse Event Reporting System. Experts do analysis and comb through the submissions to identify and follow-up on adverse reactions that might be due to vaccines. A report does not necessarily mean a vaccine caused the event.
For some new vaccines, the CDC may also initially use a tool called v-safe, which was first used for the COVID-19 vaccines. If v-safe is available, a person enrolled in the program is asked via text message or email how they are feeling at various times after being vaccinated.