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Vaccines, Thimerosal, and Neuropsychological Outcomes

The lack of any consistent association between mercury exposure and neurodevelopmental outcome is very reassuring.

Is early exposure to thimerosal, a mercury-containing preservative in vaccines, associated with neurodevelopmental deficits in children, specifically autism? In a U.S. multisite observational cohort study, investigators examined thimerosal exposure during the first 7 months of life and 42 neuropsychological outcomes at age 7 to 10 years in 1047 children with complete vaccination records. Estimates of mercury exposure included the mothers’ exposure to vaccines and immunoglobulins during pregnancy, and prenatal fish consumption.

Cumulative exposure to mercury from thimerosal from birth to 7 months ranged from 0 to 187.5 µg. After controlling for numerous confounding variables (including maternal IQ, socioeconomic indicators, and home environment), no consistent association was found between mercury exposure and any neuropsychological outcome. Among girls, higher mercury exposure from birth to 7 months was associated with better performance on two outcome measures. Among boys, greater exposure to mercury was associated with better word and letter recognition but poorer behavior regulation ratings by parents. Results were similar for mercury exposure during pregnancy and during the first 28 days of life.

Comment: Ongoing assessment of vaccine safety is critical. Although this study did not assess the relation between mercury exposure and the development of autism specifically, the lack of any consistent association between exposure and neurodevelopmental outcomes is reassuring.

— Howard Bauchner, MD

Published in Journal Watch General Medicine September 26, 2007

Citation(s):

Thompson WW et al. Early thimerosal exposure and neuropsychological outcomes at 7 to 10 years. N Engl J Med 2007 Sep 27; 357:1281.

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