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future research directions, the Immunization Safety Review Committee has been asked to assess not only the scientific plausibility of the hypothesized association but also the significance of the issue in a broader societal context. The plausibility assessment has two components: (1) an examination of the causal relationship between the vaccine and the adverse event, and (2) an examination of any pathogenic mechanisms that support the hypothesis. The significance assessment addresses such considerations as the burden of the adverse health event in question, the burden of disease that the vaccine prevents, and the level and potential consequences of public concern about the safety of vaccine use.

The findings of the plausibility and significance assessments provide the basis for the committee's recommendations regarding public health response, immunization policy review, current and future research, and effective communication strategies for the specific immunization-safety questions.

The committee adopted the framework for assessing causality developed by the committees previously convened by the IOM (IOM, 1991, 1994a) to address questions of vaccine safety. To evaluate the hypothesis on MMR vaccine and autism, the committee collected information from several sources, including a review of the published, peer-reviewed scientific and medical literature, and commissioned a background paper reviewing the epidemiological studies of MMR vaccine and autism. The committee also held an open scientific meeting in March 2001 (see Appendix B) to review the current understanding of the etiology and epidemiology of autism and on-going investigations regarding the MMR vaccine and autism hypothesis.

THE HYPOTHESIZED RELATIONSHIP BETWEEN MMR AND AUTISM

Autism is a complex and severe developmental disorder characterized by impairments of social interaction, impairments in verbal and nonverbal communication, and restricted or repetitive and stereotyped patterns of behaviors and interests (APA, 1994; Filipek et al., 1999). Over time, research has identified subtle differences in the onset and progression of autistic symptoms. The term “autistic spectrum disorders” (ASD), synonymous with “pervasive developmental disorders” (PDD), refers to a continuum of related cognitive and neurobehavioral disorders that reflects the heterogeneity of these symptoms. ASD includes autistic disorder, childhood disintegrative disorder, Asperger's syndrome, Rett's syndrome, and pervasive developmental disorder not otherwise specified (PDD-NOS or atypical autism). While the primary deficits are similar for all of these disorders, patients vary in the severity of their symptoms and level of cognitive impairment. Although Rett's syndrome is included in the diagnostic category of ASD, it is considered by many to be a distinct neurologic disorder and this diagnosis is not included in most research which has evaluated the association of the MMR vaccine with autism. In this report, the terms “autism,” “autistic,” and “autistic spectrum disorders” are used interchangeably to refer to this broader group of pervasive developmental disorders. The term “autistic disorder” refers to



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