In summary, the evidence of an association between autism and the overall immunization schedule is limited both in quantity and in quality and does not suggest a causal association. The committee found the literature to be most useful in suggesting study designs that might be adapted and extended for the committee’s core task of suggesting further research.
Other Neurodevelopmental Disorders
Forty-one papers concerning a relationship among immunizations, immunization schedule, or vaccines and learning disorders, communication disorders, developmental disorders, intellectual disability, attention deficit disorder, disruptive behavior disorders, tics, and Tourette’s syndrome were identified via an Ovid MEDLINE database search. This list was reduced to eight papers after use of the exclusion criteria described above, including exclusion of papers on vaccines not currently recommended for administration to children under age 6 years. After an initial review, five of the papers were believed to focus on some aspect of the immunization schedule and were selected for more in-depth review. Each of these five studies focused on possible adverse effects of thimerosal (given via different schedules). Importantly, with the exception of the influenza vaccine, since 2001 thimerosal has been either removed from or substantially reduced in amount in vaccines given to U.S. children under 6 years of age. Although thimerosal is no longer a component of U.S. childhood vaccines, these studies may suggest methods to study variations due to use of alternative schedules, or to changes to the recommended immunization schedule made over time. The committee identified a sixth study through its second search effort.
A study conducted by Tozzi et al. (2009) in Italy also evaluated the effects of different doses of thimerosal during infancy on neurodevelopmental outcomes. These investigators conducted a late follow-up evaluation at 10 to 12 years of age of subjects who were initially enrolled in a study of the efficacies of two formulations of pertussis vaccine that contained different amounts of thimerosal. Twenty-four neurodevelopmental outcomes were measured via 11 standardized tests. Only two statistically significant differences, which were believed not to have been clinically significant, were noted in the female subjects. Specifically, girls with higher thimerosal exposure had lower mean scores in the Boston Naming Test and on finger tapping with the dominant hand. Given the large number of comparisons, these significant differences could be attributable to chance. In this study, the cumulative dose of thimerosal was low compared with the doses that had been used in the United States.
In a cohort study of 1,047 subjects enrolled in three MCOs as part of the VSD, Thompson et al. (2007) evaluated the effects of cumulative exposure to thimerosal on 42 neurodevelopmental outcome measures (excluding