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THIMEROSAL-CONTAINING VACCINES 65 pothesis that mercury exposure had a direct role in an increasing incidence of autism in the 1990s. As noted in previous IOM reports (IOM, 1994 a,b, 2001), a positive eco- logical correlation constitutes only weak evidence of causality, and additional research would be needed to establish a causal association. In addition, the ap- propriateness of the transformation of the cross-sectional autism-caseload data into birth-cohort prevalence data, as was done in this analysis, is questionable. The data used in this analysis are cross-sectional and reflect the number of chil- dren with a diagnosis of autism who are registered in the California Develop- mental Services system. Transforming the cross-sectional data by dividing by birth cohort, creates the artificial impression of a cohort effect when the data are cross-sectional in nature (Fombonne, 2001a). Furthermore, the authors of a re- port on the California autism caseload data stress that their study was not de- signed to measure trends in autism and that the data should therefore be inter- preted with caution (California Department of Developmental Services, 1999~. The analytical value of the data is limited by the inability to account for any changes over time in diagnostic concepts, case definitions, or age of diagnosis (Fombonne, 2001a). As a result, the committee cannot assess trends in the prevalence of autism from these data. The committee concludes that this unpub- lished ecological analysis is uninformative with respect to causality. CAUSALITY ARGUMENT A number of case reports have been submitted through VAERS and directly to the committee asserting an association between exposure to thimerosal- containing vaccines and neurological outcomes, autism in particular. Case re- ports are very useful in hypothesis generation and identifying areas for investi- gation, but are rarely useful in establishing causality. The case reports reviewed by the committee are uninformative with respect to causality. There are no published epidemiological studies that examine the potential association between exposure to thimerosal-containing vaccines and neurode- velopmental disorders. Unpublished studies reviewed by the committee provide inconclusive evidence of a potential association between exposure to thimerosal- containing vaccines and certain neurological developmental disorders. Among the unpublished reports, only one an examination of the hypothesis using data from the VSD (Verstraeten, 2001) is from a controlled epidemiological study. The committee, like other experts convened by the CDC, has several concerns about this study. These include: the inexactness of the diagnoses and inconsis- tencies across clinicians, clinics, and study sites, variation in results between study sites, indicative of potential ascertainment or health-care-seeking bias, potential bias due to disenrollment, the small effect sizes, the inconsistent dura- tion of observation, insufficient power to detect less common diagnoses, lack of information on prenatal exposures to mercury, and lack of a consistent dose-