Immunization Science

Thimerosal Exposure and Developmental Disorders

The article

Thimerosal Exposure in Infants and Developmental Disorders: A Prospective Cohort Study in the United Kingdom Does Not Support a Causal Association. Heron J, Golding J, and the ALSPAC Study Team. Pediatrics 2004;114: 577-583.

The question

Did thimerosal exposure cause developmental disorders in young children?

The study

This study analyzed data about the children born to 14,541 women from Avon, England who were prospectively enrolled and who had an expected date of delivery between April 1, 1991, and December 31, 1992.

The researchers collected information on childhood behavior and development from questionnaires administered to the mothers. Information about the children’s immunizations was obtained from the Bristol-based Child Health Surveillance Database (NHS Public Health Network).

They evaluated 12, 956 children who were singleton births that survived to at least a year of age and on whom they had valid information on thimerosal exposure. They defined the mercury exposure for each child according to the number of diphtheria/tetanus/pertussis (DTP) or DT doses received by 3 months and by 4 months of age.

The researchers also examined other factors that could affect childhood behavior and development, such as birth weight, gestation, gender, maternal education, parity, housing tenure, maternal smoking, breastfeeding, and ethnic origins. Subsequently, they also considered the effects of maternal fish consumption (another source of mercury) on the children studied.

The findings

When taking into account only thimerosal exposure, the unadjusted results of this study suggested that thimerosal exposure had a beneficial effect—in other words, children with a higher exposure to thimerosal had less behavioral and developmental problems than children with a lower exposure—those who received fewer doses of thimerosal-containing vaccines.

However, after adjusting for the other factors, only 1 result of 69 evaluations suggested that poor social behavior at 47 months of age could have been associated with thimerosal exposure at 3 months of age. However, the results of 8 other evaluations suggested a beneficial effect, even after adjusting for other factors.

The relevance/bottom line

This study shows no evidence of any harmful effect of thimerosal-containing vaccines on neurologic or psychological outcome.

NNii’s comment

The findings of this—and an increasing number of other studies—should provide reassurance to families and others that there continues to be no evidence of a relationship between developmental problems and thimerosal-containing vaccines.