A study of 1,047 children found no evidence of a causal association between exposure to ethylmercury from the preservative thimerosal in vaccines during the prenatal, neonatal, or first 7 months of life and neuropsychological outcomes at ages 7–10 years, said the authors.
“The associations that we detected were small, almost equally divided between positive and negative effects, and mostly sex-specific,” William W. Thompson, Ph.D., an epidemiologist in the National Center for Immunizations and Respiratory Diseases, at the Centers for Disease Control and Prevention (CDC), and his coauthors wrote in the New England Journal of Medicine.
“The study results were very reassuring,” Dr. Anne Schuchat, director of the CDC's National Center for Immunizations and Respiratory Diseases, said during a CDC-sponsored teleconference. “The bulk of the study found very, very similar performance in children who were exposed to high amounts of thimerosal and children exposed to low or no thimerosal and suggests the higher thimerosal content that vaccines had back in the 1990s did not lead to harmful effects in children in performance on standardized testing at ages 7–10.”
She noted that parental reports of tics were not associated with any higher exposure of thimerosal but based on evaluator observations during the testing; motor and phonic tics were increased by about twofold among boys with higher thimerosal exposure from birth to 7 months, an association that was not seen in girls. Because of similar findings in two previous studies, this issue is being evaluated further with CDC experts in developmental disability and consultations with outside pediatric neurologists, she said.
This was not an autism study. No measures of autism were included in the testing because the CDC is conducting a separate case-control study of autism and mercury exposure, the results of which are expected in the next year, Dr. Schuchat said.
In the cohort study, children enrolled in four HMOs, who were born between January 1993 and March 1997, were administered a wide range of standardized tests that measured 42 neuropsychological outcomes, including speech and language indexes, verbal memory, achievement, fine-motor coordination, visuospatial ability, attention, behavior regulation, tics, and general intellectual functioning, between the ages of 7 and 10 years. Estimates of their exposure to mercury from thimerosal in vaccines and immunoglobulins during the prenatal period, neonatal period (birth to 28 days) and the first 7 months (1–214 days) were based on computerized and personal immunization records, medical records, and parent interviews (N. Engl. J. Med. 2007;357:1281–92).
Between birth and 7 months, the median cumulative exposure to ethylmercury from thimerosal among the children was 112 mcg; 9% of the children were exposed to 62.5 mcg or less of mercury, and 25% had cumulative exposures of 150 mcg or more. (There was no exposure to a thimerosal-containing vaccine or immune globulin during the first 7 months of life in 1.5% of the children.) During the first 28 days, 30% of the children were not exposed to thimerosal and 1.6% had been exposed to more than 12.5 mcg of mercury in hepatitis B and immune globulins. Fewer than 11% of the children had been exposed to thimerosal prenatally when their mothers received vaccinations and immune globulins during pregnancy.
There were “few significant associations” among performance on neuropsychological test results and exposure to mercury in vaccines and immune globulins administered prenatally or up through 7 months of age, the authors reported.
These included a significant association between prenatal exposure to higher ethylmercury levels and a better performance on one measure of language and poorer performance on one measure of attention and executive functioning. Increasing levels of mercury exposure from birth to 7 months were associated with significantly better performance on one measure of fine-motor coordination and on one measure of attention and executive functioning.
Increased mercury exposure during the first 28 days was associated with worse performance on one measure of speech articulation and better performance on one measure of fine-motor coordination. Among the children overall, there was no association between neonatal exposure to mercury from thimerosal and total IQ, while among boys, there was a significant positive association with performance IQ, and among girls, there was a significant negative association with verbal IQ.
“Although the effect sizes were very small, the speech-articulation findings among all children and the lower verbal IQ findings among girls suggest a possible adverse association between neonatal exposure to mercury and language development,” the authors said. “Conversely, the finding of higher scores on the performance IQ tests in boys makes it difficult to draw general conclusions about possible effects of neonatal mercury exposure from vaccines and immune globulins on intellectual abilities.”