Why It’s Important to Monitor Vaccine Safety

John Iskander, MD, MPH, acting director, Immunization Safety Office, and Robert T. Chen, MD, MA, blood safety specialist, Epidemiology Branch, NCHHSTP,
wrote a chapter in the new book

Infectious Disease Surveillance to explain why it is important to monitor vaccine safety.

Rare Reactions.
The most important reason is to detect rare reactions. Although vaccines are tested extensively before they are licensed for use in the United States, not enough people are included in the tests to detect reactions that happen only rarely. If serious reactions are found when the vaccine is in widespread use, the vaccine may be withdrawn.

Higher Risk Groups.
Vaccine safety monitoring also makes sure new vaccines are safe for groups such as the elderly, those with chronic medical conditions, and pregnant women. Vaccine trials may deliberately exclude members of these groups.

Public Confidence in Vaccines.
Monitoring vaccine safety also helps to maintain public confidence needed to keep enough people vaccinated to prevent disease outbreaks.

How We Know If Vaccines Are Safe
Vaccine safety cannot be measured directly. Instead, it is estimated by the number of "adverse events" reported. An adverse event is "... a medical incident that takes place after an immunization ... and is believed to be caused by the immunization." Adverse events include—

True reactions to the vaccine.
Events that would have occurred even if the person had not been vaccinated (unrelated coincidences).
Reactions related to mistakes in vaccine preparation, handling, or administration.
Events that cannot be related directly to the vaccine; their cause is unknown.

A formal scientific study usually is required to distinguish between coincidences and true reactions. It is rarely possible to say for sure whether a vaccine caused a specific adverse event.