How Vaccines Are Tested and Monitored

Vaccines are one of the greatest success stories in public health. Vaccines have ended smallpox, nearly ended polio, and reduced outbreaks of measles, pertussis, and other illnesses to an all-time low.

Vaccines are our best defense against infectious diseases, but no vaccine is 100% safe or effective. People react differently to vaccines, so some people have adverse events or are not protected from the disease. Scientists are trying to develop safer, more effective vaccines.

Testing Vaccines Before Licensing

Vaccines must be licensed by the U.S. Food and Drug Administration (FDA) before they can be used in the United States. Before the FDA approves a license, vaccines are tested extensively to ensure they are safe. First, computers are used to predict how the vaccine will work. Then researchers test the vaccine on animals.

When the vaccine completes these laboratory tests successfully, the FDA allows it to be tested on people during "clinical trials." People who participate in a clinical trial understand the risks. They choose to get the vaccine and be tested to find out if the vaccine is safe and effective.

If the clinical trials show the vaccine works safely, the manufacturer asks the FDA for a license. The FDA reviews the clinical trial results and the proposed label for the vaccine. The FDA also inspects the plant where the vaccine will be made and reviews the process for making the vaccine. When the FDA is satisfied it is safe, the vaccine is licensed for public use. The licensing process can take 10 years or longer.

Monitoring Vaccines After Licensing

After a vaccine is licensed for public use, its safety is monitored. The FDA requires all manufacturers to submit samples from each vaccine lot before its release. Manufacturers must give the FDA their test results for vaccine safety, potency, and purity.

Vaccine Adverse Event Reporting System (VAERS)

Rare side effects and delayed reactions may not happen until the vaccine is given to millions of people. The Vaccine Adverse Event Reporting System (VAERS) takes reports of health problems that happen after vaccination so scientists can find out if a vaccine is causing a health problem.

Anyone can report a health problem in VAERS. Just because a health problem is reported to VAERS does not mean it was caused by a vaccine. Only scientific research can determine if a vaccine really caused the health problem, or if it was just a coincidence.

People who report a health problem to VAERS are asked if the problem led to hospitalization, life-threatening illness, disability, or death. These events are classified as "serious," and are often subject to further study.

Vaccine Safety Datalink (VSD) Project

Due to the limitations of VAERS, CDC developed the Vaccine Safety Datalink (VSD) project in 1990. It stores complete medical records for 5.5 million people who are members of participating managed care organizations, and watches them for possible side effects of vaccination.

The Future

Monitoring vaccine safety will become more important as new vaccines are developed and added to complicated immunization schedules. Scientists may also find new ways of giving vaccines, such as edible vaccines and needle-free injections. Vaccines will remain our best tool for preventing disease in the future. Reporting systems like VAERS will continue to be used to monitor health problems, so vaccines can continue to be held to very high standards of safety.