Does Thimerosal Affect Neuropsychological Development?

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Summary and Comment

Does Thimerosal Affect Neuropsychological Development?

No consistent association was found between early childhood exposure to thimerosal-containing vaccines and neuropsychological development.

Thimerosal, used as a preservative for vaccines since the early 1930s, is 50% mercury by weight. Some have questioned whether the inclusion of thimerosal in vaccines administered to young infants could lead to mercury toxicity and impair neurologic development. To address this concern, researchers conducted a multicenter cohort study among children aged 7 to 10 years with no history of confounding neurologic conditions. The investigators reviewed the children’s medical records to determine exposure to thimerosal-containing vaccines during three time periods (prenatally, the first 28 days of life, and the first 7 months of life) and conducted standardized tests to assess 42 neuropsychological outcome measures, including speech and language function, verbal memory, fine motor coordination, visuospatial ability, attention and executive-functioning tasks, behavior regulation, tics, and general intellectual function.

Among the 1047 participants, the median cumulative exposure to mercury from thimerosal was 112.5 µg (range, 0–188 µg) between birth and age 7 months. Overall, no consistent pattern of association was seen between neuropsychological function and mercury exposure. Higher levels of mercury exposure were associated with only minor differences in individual test findings. Both positive and negative effects were observed, with the results differing across the three exposure periods and between the sexes.

Comment: Although these results do not exclude the possibility that thimerosal might subtly affect some aspects of neuropsychological development in children, the pattern of correlations seen in this study is suggestive of chance findings. The researchers did not look for an association between thimerosal exposure and autism because the CDC is currently conducting a case-control study on that issue.

— Richard T. Ellison III, MD

Published in Journal Watch Infectious Diseases September 26, 2007