Vaccines go through a lot of testing.

Why? Because they are meant for healthy people and babies, vaccines often go through even more testing than other medical products.

Vaccines are tested in labs and in hundreds to thousands of human volunteers before being approved for the public.

This testing process takes many years, often more than a decade.

If new ingredients are added to an existing vaccine, the testing process is repeated.

If a vaccine is meant to be given at the same time as another vaccine, additional tests are done to make sure the two vaccines are safe and effective when given together.

Once in use, vaccines continue to be closely monitored for safety.

The government requires vaccine makers to test that every batch meets high quality standards.

In the U.S., there are several systems in place to track any unexpected side effects from vaccines. Some of these systems rely on doctors and patients to report suspected side effects. Other systems monitor health data. The government also has vaccine experts who consult directly with doctors if there is a question about vaccine safety.

If a new, serious side effect is discovered, the government quickly notifies doctors and the public. In some cases, the vaccine will no longer be used.

Because vaccines are given to millions of people every year and are so closely tracked, it is very unlikely that a serious side effect would go unnoticed for long.