Neuropsychological Performance 10 Years After Immunization in Infancy With Thimerosal-Containing Vaccines

Neuropsychological Performance 10 Years After Immunization in Infancy With Thimerosal-Containing Vaccines

PEDIATRICS Vol. 123 No. 2 February 2009, pp. 475-482

Abstract:

OBJECTIVE. Thimerosal, a mercury compound used as a preservative in vaccines administered during infancy, has been suspected to affect neuropsychological development. We compared the neuropsychological performance, 10 years after vaccination, of 2 groups of children exposed randomly to different amounts of thimerosal through immunization.

METHODS. Children who were enrolled in an efficacy trial of pertussis vaccines in 1992–1993 were contacted in 2003. Two groups of children were identified, according to thimerosal content in vaccines assigned randomly in the first year of life (cumulative ethylmercury intake of 62.5 or 137.5 µg), and were compared with respect to neuropsychological outcomes. Eleven standardized neuropsychological tests, for a total of 24 outcomes, were administered to children during school hours. Mean scores of neuropsychological tests in the domains of memory and learning, attention, executive functions, visuospatial functions, language, and motor skills were compared according to thimerosal exposure and gender. Standard regression coefficients obtained through multivariate linear regression analyses were used as a measure of effect.

RESULTS. Nearly 70% of the invited subjects participated in the neuropsychological assessment (N = 1403). Among the 24 neuropsychological outcomes that were evaluated, only 2 were significantly associated with thimerosal exposure. Girls with higher thimerosal intake had lower mean scores in the finger-tapping test with the dominant hand and in the Boston Naming Test.

CONCLUSIONS. Given the large number of statistical comparisons performed, the few associations found between thimerosal exposure and neuropsychological development might be attributable to chance. The associations found, although statistically significant, were based on small differences in mean test scores, and their clinical relevance remains to be determined.

 

Responses:

Il Mercurio and the AAP

An Addendum to February’s “First Read”

An Addendum to February’s “First Read”

It has been brought to my attention that in the study on thimerosal briefly highlighted below, I noted that groups did or did not get thimerosal and had similar results in terms of neuropsychological developmental outcomes. Reading of this study will indicate that both groups studied actually did have thimerosal in their vaccines, one group having 62.5 micrograms cumulative intake and the other 137.5 micrograms cumulative intake. While the amount of thimerosal in the lower group studied in Italy is less (according to the author of this study Dr. Tozzi) than the small amounts used in this country, I do want to correctly indicate there was no group studied that received no thimerosal whatsoever in their vaccines. The results of this study suggesting essentially minimal (if any) differences in developmental outcomes remains as stated–although a limitation as noted by Dr. Tozzi is that there was no comparison group with zero exposure to thimerosal. I appreciate the readers who have brought this to my attention so that I could more accurately clarify my interpretation of this study. Please read this article for yourself to learn more.

Truth or consequences about mercury

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