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In 1999, the AAP and the U.S. Public Health Service suggested that vaccine manufacturers remove thimerosal preservatives from vaccines to minimize potential mercury toxicity to the developing brain. In this study, investigators examined pre- and postnatal mercury exposure and neuropsychological outcomes in 1047 children at age 7 to 10 years at four HMOs.
Data were collected from complete vaccination records, maternal interviews, and 3-hour neuropsychological assessments of 42 motor and developmental tasks. Estimates of mercury exposure included mothers’ pre- and postnatal exposure to immunoglobulin and vaccines and prenatal fish consumption. Overall, no consistent association was found between pre- or postnatal thimerosal exposure and neuropsychological outcome. A few positive and negative sex-specific associations were noted.
Thompson WW et al. Early thimerosal exposure and neuropsychological outcomes at 7 to 10 years. N Engl J Med 2007 Sep 27; 357:1281.
Comment
These results are reassuring for parents whose children were immunized before thimerosal was removed from vaccines. The small potpourri of associations probably reflects the large number of outcomes examined, because no plausible biologic explanation exists for both positive and negative effects. This study did not address a link between thimerosal and autism, but those findings will be reported by the CDC in a separate study. For now, we can reassure parents that mercury has been removed from vaccines, that no clear adverse outcomes have been associated with thimerosal exposure, and that we will continue to assess vaccine safety.