Neuropsychological performance 10 years after immunization in infancy with thimerosal-containing vaccines.
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 microg), 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.
Tozzi
Alberto Eugenio
AE
Epidemiology Unit, Bambino Gesù Hospital, Piazza S. Onofrio 4, 00165 Rome, Italy. alberto.tozzi@gmail.com
Bisiacchi
Patrizia
P
Tarantino
Vincenza
V
De Mei
Barbara
B
D'Elia
Lidia
L
Chiarotti
Flavia
F
Salmaso
Stefania
S
D002648:Child; D002658:Developmental Disabilities / Q000139:chemically induced* / Q000175:diagnosis*; D005260:Female; D006801:Humans; D007223:Infant; D008297:Male; D009483:Neuropsychological Tests; D011310:Preservatives, Pharmaceutical / Q000008:administration & dosage* / Q000009:adverse effects*; D013849:Thimerosal / Q000008:administration & dosage* / Q000009:adverse effects*; D013997:Time Factors; D014612:Vaccines
D011310:Preservatives, Pharmaceutical; D014612:Vaccines; D013849:Thimerosal