autism). The subjects were between 7 and 10 years of age. Immunization status was retrospectively assessed, and the assessment included exposure to thimerosal both prenatally (via maternal immunization or immunoglobulin administration) and then during the first 7 months of life. Few significant associations between cumulative thimerosal exposure and a particular neurodevelopmental outcome were noted. These associations were few in number and were equally divided between positive and negative effects. Most were gender specific. For example, in boys, higher exposure to thimerosal prenatally was associated with a higher score on the Stanford-Binet copying test and a lower score on the Wechsler Intelligence Scale for Children III (WISC-III) digit-span test of backward recall. In girls, higher thimerosal exposure at between birth and 7 months of age was associated with a better performance on the Grooved Pegboard Test in the nondominant hand as well as on the WISC-III digit-span test of backward recall. Although this study was limited by only a 30 percent participation rate, which may have resulted in selection bias, it failed to demonstrate a causal association between early exposure to mercury via thimerosal-containing vaccines or immunoglobulins and neurodevelopment.
Smith and Woods (2010) used secondary data from the VSD cohort study of Thompson et al. (2007) to determine if on-time immunization by 1 year of age was associated with neuropsychological outcomes. The researchers performed two analyses using immunization and outcomes data from the VSD. The first analysis compared children who had received all vaccinations on time with those who had not. Complete immunization was defined as having received within 30 days of the recommended age at least two doses of HepB, three doses of DTaP, three doses of Hib, and two doses of polio vaccine (referred to as the 2:3:3:2 series) during the first year of life. The second analysis stratified children into five groups by age at the time of completion of the 2:3:3:2 series. Children with on-time immunizations consisted of those who received at least 10 vaccinations in the first 7 months of life, whereas the least vaccinated group comprised those who had received less than seven vaccine doses of any type during the same time period. Using the outcomes data obtained from the research of Thompson et al. (2007), Smith and Woods (2010) found that children who had received their immunizations on time and also those who had received at least 10 doses did not have better neuropsychological outcomes in this study than those who had received fewer doses, and no significant differences were found between those who received the least vaccines and those with the greatest vaccine exposure during the first 7 months of life.
In a cohort study conducted in Brazil, Marques et al. (2007) evaluated the effects of thimerosal exposure during the neonatal period on neurodevelopment measured by use of the Gesell battery of tests at 6 months of age. In their study, 84 infants were immunized with a thimerosal-containing