Immunization to protect children and adults from infectious diseases is one of the greatest achievements of public health. Immunization is not without risks, however. It is well established, for example, that the oral polio vaccine on rare occasion has caused paralytic polio and that vaccines sometimes produce anaphylactic shock. Given the widespread use of vaccines, state mandates requiring vaccination of children for entry into school, college, or day care, and the importance of ensuring that trust in immunization programs is justified, it is essential that safety concerns receive assiduous attention.

At the request of the sponsoring agencies, the Centers for Disease Control and Prevention (CDC) and the National Institutes of Health (NIH), the Institute of Medicine (IOM) established the Immunization Safety Review Committee to evaluate the evidence on possible causal associations between immunizations and certain adverse outcomes, and to then present conclusions and recommendations. The committee’s mandate also includes assessing the broader significance for society of these immunization safety issues.

The specific vaccine safety hypotheses issues examined by the committee are determined by the Interagency Vaccine Group (IAVG), whose members represent several units of the Department of Health and Human Services: the CDC’s National Vaccine Program Office, National Immunization Program, and National Center for Infectious Diseases; the NIH’s National Institute of Allergy and Infectious Diseases; the Food and Drug Administration (FDA); the Health Resources and Services Administration’s National Vaccine Injury Compensation Program; and the Centers for Medicare & Medicaid Services. The IAVG also includes representation from the Department of Defense and the Agency for International Development. The committee has issued seven previous reports on vaccine safety issues over the three-year study period (2001-2003). This eighth and final report from the committee examines the hypothesis that vaccines, specifically the measles-mumps-rubella (MMR) vaccine and vaccines containing the preservative thimerosal, cause autism. In its first two reports that were published in 2001, the committee examined the hypothesized causal association between the MMR vaccine and autism, and thimerosal-containing vaccines and neurodevelopmental disorders, respectively (IOM, 2001a,b). The IAVG asked the committee to revisit the hypothesized causal association between vaccines and autism in its final report in order to update its conclusions and recommendations based on the significant number of studies that have been undertaken in the last three years.

The committee begins from a position of neutrality regarding the specific immunization safety hypothesis under review. That is, there is no presumption that a specific vaccine (or vaccine component) does or does not cause the adverse event in question. The weight of the available clinical and epidemiologic evidence determines whether it is possible to shift from that neutral position to a finding for causality (“the evidence favors acceptance of a causal relationship”) or against causality (“the evidence favors rejection of a causal relationship”). The committee does not conclude that the vaccine does not cause the adverse event

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