A vaccine is a medicine that helps your immune system learn to fight off a particular disease. It usually contains a weakened (attenuated) form of a virus or bacteria, or sometimes a toxin produced by that organism.
Laboratory researchers funded by the government, academic foundations or industry can take two to four years or more to find a natural or manufactured antigen that can trigger a protective response in humans. This is called the pre-clinical phase of vaccine development.
Once a vaccine is produced, it undergoes rigorous testing in animals and then in people for safety and effectiveness. Encouraging results prompt Phase III clinical trials with thousands of volunteers. Once vaccines are approved by the FDA based on their efficacy and safety, they are licensed for use in the United States. Vaccines continue to be tested after they are approved through a program known as VAERS, which collects information on any problems that vaccines cause in people.
After a person receives a vaccine, his or her immune cells develop antibodies against the germs it protects against. These antibodies act like a “wanted poster” that tells other immune cells to hunt and destroy the invaders. Immunity from a vaccine lasts a lifetime, although some people require booster doses to maintain it.
Most vaccines have few side effects, and serious reactions are extremely rare. The best way to reduce the chance of a reaction is to follow your physician’s recommended vaccine schedule.