Early exposure to mercury-containing vaccines has no consistent effect on neuropsychology
Original article
Thompson WW et al. (2007) Early thimerosal exposure and neuropsychological outcomes at 7 to 10 years. N Engl J Med 357: 1281–1292 PubMed
Concerns have been expressed about the potentially negative neuropsychological effects of the mercury-containing preservative thiomersal (thimerosal) in pediatric vaccines; however, a recent cohort study found no evidence to indicate a causal relationship between deficits in neuropsychological functioning at 7–10 years and early exposure to thiomersal-containing vaccines.
The study used standardized tests to assess 42 neuropsychological outcomes in 1,047 children aged 7–10 years; autism–spectrum disorders were not assessed. Each child's exposure to mercury from thiomersal was determined from computerized immunization records, medical records, parent immunization records and parent interviews. The median cumulative exposure to mercury from thiomersal from birth to 7 months was 112.5 
g (range 0–187.5 g); 16 children had no thiomersal exposure during this period. After adjustment for potential confounding factors, few significant associations were found between test performance and exposure to thiomersal. Those associations that were detected were small, almost equally divided between positive and negative effects, and mostly sex-specific. The degree of mercury exposure from birth to 7 months correlated positively with better performance on one test of fine motor coordination in both boys and girls and in one test of achievement (letter and word identification) in boys. A possible adverse effect of neonatal (birth to 28 days) mercury exposure on language development was noted; however, in boys there was a positive association between neonatal exposure to mercury and performance IQ. The authors suggest that the few significant associations observed might simply be chance findings attributable to the large number of statistical tests performed.
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