Why it's 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 If serious reactions are found when the vaccine is in widespread use, the vaccine may be withdrawn.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.
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 yet been vaccinated (unrelated consequences.
- 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.
Almost all national immunization programs have a system for reporting adverse events. The United States Vaccine Adverse Event Reporting System (VAERS) and the Canadian Adverse Events Following Immunization Surveillance System (CAEFISS) are examples. People who report a vaccine reaction to VAERS or CAEFISS are asked if the reaction led to hospitalization, life-threatening illness, disability, or death. These events are classified as “serious,” and are often subject to further study that yields important information.
For example, reports to CAEFISS identified a common illness among people who received flu vaccines from one Canadian manufacturer in one season. Equally important, such systems have supported the safety of new vaccines such as the new meningococcal B and C vaccines in New Zealand and the United Kingdom.
Other monitoring programs include the Vaccine Safety Datalink (VSD) project and the Clinical Immunization Safety Assessment (CISA) Network. The VSD project is a collaboration between CDC's Immunization Safety Office and eight large managed care organizations that allows for planned vaccine safety studies as well as quick investigation of possible issues. The CISA Network of six medical research centers conducts clinical research on vaccine-associated health risks.
Vaccine safety monitoring becomes more important with new vaccines, expanded vaccine recommendations, and new global immunization initiatives. Reporting systems like VAERS will continue to be used to monitor adverse events, so vaccines can continue to be held to very high standards of safety.
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CDC Connects reporter Yolanda Barnes contributed to this article.
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