Immunization Science

Autism Cases in California After Removal of Thimerosal

The article

Continuing increases in autism reported to California’s Developmental Services System: Mercury in retrograde. Schechter R and Grether JK. 2008. Arch Gen Psychiatry 65(1): 19-24.

Explanatory note: Autism and similar disorders (grouped within the term autism spectrum disorders or ASD) have been increasingly recognized over the past 20-30 years. Some people have speculated that exposure to thimerosal preservative in vaccines was the major cause of the increase in cases of ASD, although many careful epidemiologic studies have not supported such an association. Beginning in 1999, the amount of thimerosal in vaccines recommended for routine use in young infants began to be reduced and the vaccine supply for infants in the United States was largely thimerosal-preservative free by the end of 2002. If thimerosal-containing vaccines were a major cause of ASD, a sharp decline in cases would now be expected to be evident.

The question

Was the reduction of thimerosal in vaccines administered to infants, associated with a decrease in cases of autism reported to California’s Developmental Disabilities System (DDS)?

The study

The study examined the prevalence of ASD in children 3 years of age and older reported to the California DDS using client reports for the years 1989 through March, 2007. Prevalence was calculated as the number of cases reported divided by the number of live births for the year as well as for California age-specific populations. The data were also examined for 3 to 5 year old children from DDS quarterly reports.

The findings

The prevalence of children with autism born from 1989 through 2003 increased each year (and for each age up to 10 years). This was true whether the data were examined by year of birth or age group. Specifically, the prevalence at ages 3 to 5 years increased for each birth year since 1999, during the period when exposure to thimerosal preservative in vaccines began to be reduced.

The relevance/bottom line

If thimerosal in vaccines were a major cause of autism, the prevalence of autism would be predicted to fall as exposure was reduced. These data show no decrease—in fact they show a continued increase—in cases of autism in California since the reduction of thimerosal in pediatric vaccines.

NNii’s comment

This ecologic study had several limitations: it may not have included all children with autism in California; because of changed client definitions in 2003, it might have underestimated cases in the later years compared to earlier years; and, the diagnoses were not corroborated with medical records. Despite these caveats, this study adds to the now robust set of data rejecting the hypothesis that thimerosal-containing vaccines are a major cause of autism.