HepB either on the day of birth or later in the neonatal period (between days 2 and 4 of life). Before the neurodevelopmental assessments at 6 months of age, these infants also received additional doses of vaccines containing thimerosal (two doses of HepB and three doses of DTP). The researchers did not report any difference in neurodevelopmental measures between the two groups. In addition to the small sample size, this research focused on a minimal alteration in the immunization schedule that may have been so minor that an effect on neurodevelopment would not be expected.
In a longitudinal study of 14,000 infants in the United Kingdom, Heron et al. (2004) evaluated the relationship between cumulative exposure to thimerosal and several neurodevelopmental outcomes, including behavioral difficulties, tics, deficits in speech and fine motor development, and other “special needs.” At the time of this study, thimerosal-containing vaccines were administered in the United Kingdom at 2, 3, and 4 months of age, which represents an accelerated schedule of exposure compared with the schedule used in the United States. This study evaluated 69 specific behavioral and developmental outcomes via questionnaires that were sent to the parents of children born over a 15-month interval during 1991 and 1992. Only one outcome (poor prosocial behavior) was found to be associated with cumulative thimerosal exposure at 3 months of age. Interestingly, this study demonstrated that adverse neurodevelopmental outcomes were less likely in children who had higher thimerosal exposures.
In another VSD study, Verstraeten et al. (2003) also evaluated the association between the cumulative exposure to thimerosal at 1, 3, and 7 months of age and neurodevelopmental disorders such as autism, other speech and language disorders, disorders of attention, and tics. This was a large retrospective cohort study of subjects from three MCOs that participated in the VSD. In Phase 1 of the study, data from two MCOs were analyzed. A positive association between cumulative thimerosal exposure and the development of tics was found for subjects from one MCO, whereas a positive association with language delay was found for subjects from the other MCO. In Phase 2 of the study, the most common associations seen in Phase 1 were evaluated in a third MCO, and no significant associations were demonstrated. Therefore, no consistent significant association between cumulative thimerosal exposure and neurodevelopmental outcomes was found. Importantly, in no instance was a significant risk of cumulative thimerosal exposure and either autism or disorders of attention detected. This study was limited, as the investigators evaluated thimerosal only as opposed to the type of vaccine. Neurodevelopmental outcomes for the subjects were determined only by medical record designations (codes) and not by a review of the results of formal neuropsychological assessments.
In summary, the evidence regarding an association between the overall immunization schedule and other neurodevelopmental disorders is limited