The safety and effectiveness of vaccines are under constant study. Because vaccines are designed to be given routinely during well-child care visits, they must be extraordinarily safe. Safety testing begins as soon as a new vaccine is contemplated, continues until it is licensed, and is monitored indefinitely after licensure.
Before a vaccine is ever recommended for use, it’s tested in labs. This process can take several years. FDA uses the information from these tests to decide whether to test the vaccine with people.
During a clinical trial, a vaccine is tested on people who volunteer to get vaccinated. Clinical trials usually start with 20 to 100 volunteers, but eventually include thousands of volunteers. These tests can take several years and answer important questions like:
