Thimerosal Is Not Responsible for Autism

James R. Laidler, MD

It is hard to know exactly when the hypothesis that mercury could cause autism was first proposed. The idea was circulating well before 2000, the year that thimerosal—a mercury-containing preservative—was removed from children’s vaccines. In its October 2001 report, Immunization Safety Review: Thimerosal-Containing Vaccines and Neurodevelopmental Disorders, the Institute of Medicine (IOM) concluded that, while it was biologically plausible that thimerosal could cause autism, no data supported the claim that it did cause autism.

Since that time, much has happened (and not happened). The most significant thing has been, as Sherlock Holmes said in "Silver Blaze," the "dog that didn’t bark." Because manufacturers stopped using thimerosal in children’s vaccines in 2000, few children now under age four have ever received a thimerosal-containing vaccine. Even those born to Rh-negative mothers (who received at least one dose of thimerosal-containing RhoGam during pregnancy) have gotten far less thimerosal than older children. If thimerosal were a major cause of autism, we should see a drop in autism incidence, which has yet to materialize.

This non-event shouldn’t come as a surprise. Denmark mandated removal of thimerosal from vaccines in 1992. Even allowing for continued use of thimerosal-containing stocks of vaccine, Danish children were thimerosal-free by 1995. Autism prevalence in Denmark has risen in exactly the same fashion as in the United States and the United Kingdom.

A number of supporters of the thimerosal-autism connection have "corrected" the numbers of the Danish studies, manipulating the numbers in ways that would make Enron accountants blush. They can, through mathematical sleight-of-hand, turn the exponential upward curve into a downward trend, but they have yet to explain why the autism numbers in Denmark keep rising every year if the prevalence is dropping. So, no matter how many possible mechanisms are proposed, no matter how they "massage" the data, the cold, hard facts are these:

The conclusion should be inescapable.

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This page was posted on January 31, 2005.

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