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Why It’s Important to Monitor Vaccine Safety

Cover of Infectious Disease Surveillance Book

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 If serious reactions are found when the vaccine is in widespread use, the vaccine may be withdrawn.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."1 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.

Almost all national immunization programs have a system for reporting adverse events. The United States Vaccine Adverse Event Reporting System (VAERS)2 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.3 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, the Clinical Immunization Safety Assessment (CISA) Network, and the Brighton Collaboration. 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, and the Brighton Collaboration develops standard case definitions for problems following immunization as well as guidelines for data collection, analysis, and presentation.

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.


  1. World Health Organization. Immunization Safety Surveillance. Manila: Immunization Focus, Western Pacific Regional Office; 1999. Available from: Accessed May 30, 2006.

  2. Zhou W, Pool V, Iskander JK, English-Bullard R, Ball R, Wise RP, Haber P, Pless RP, Mootrey G, Ellenberg SS, Braun MM, Chen RT. Surveillance for safety after immunization: Vaccine Adverse Event Reporting System (VAERS)—United States, 1991–2001. MMWR 2003;52(ss01);1–24.

  3. Skowronski DM, Strauss B, De Serres G, MacDonald D, Marion SA, Naus M, Patrick DM, Kendall P. Oculo-respiratory syndrome: a new influenza vaccine-associated adverse event? Clinical Infectious Diseases 2003;36(6):705–713.

CDC Connects reporter Yolanda Barnes contributed to this article.

*Links to non-Federal organizations found at this site are provided solely as a service to our users. These links do not constitute an endorsement of these organizations or their programs by CDC or the Federal Government, and none should be inferred. CDC is not responsible for the content of the individual organization Web pages found at these links.

Page last reviewed: June 23, 2008
Page last updated: January 11, 2008
Content source: Immunization Safety Office

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