THIMEROSAL AND AUTISM….Robert F. Kennedy Jr. and thousands of distraught parents to the contrary, the evidence linking thimerosal to autism has never been very strong. Thimerosal is a mercury-based preservative used in many childhood vaccines, and its connection to autism was always based primarily on a coincidence: most children get vaccinated when they’re about two years old and most autism gets diagnosed between the ages of 2-4. That, along with the fact that mercury has well-known effects on infant brain development, made thimerosal seem like a plausible culprit for what’s often been referred to as an “epidemic” of autism over the past decade.

However, despite the equivocal (at best) scientific evidence linking thimerosal to autism, conspiracy theories abounded and the issue deeply split the autism community. Firm evidence in one direction or the other, though, had to wait until now. Thimerosal was ordered removed from most childhood vaccines in 1999, and by the early 2000s children had stopped receiving virtually all thimerosal-based vaccines. If autism rates then decreased, it would be good evidence that thimerosal really had been to blame.

But that didn’t happen. Interim studies have shown no decrease in autism rates, and a study released today puts the nail in the coffin of the thimerosal story. It tracks children born in California and includes enough years of data to show pretty definitively that autism diagnoses continued to rise even after thimerosal was removed. From Science Daily:

“The estimated prevalence of autism for children at each year of age from 3 to 12 years increased throughout the study period,” the authors write…..”The prevalence at ages 3 to 5 years has increased monotonically for each birth year since 1999, during which period exposure to thimerosal has been reduced,” they continue.

….In addition to analyzing the prevalence of autism by birth year, the researchers also examined the rate among children age 3 to 5 based on quarterly reports issued by the Department of Developmental Services. Prevalence increased each quarter from January 1995 (0.6 per 1,000 live births) through March 2007 (4.1 per 1,000 live births), including after 2004, when the researchers estimate that exposure to thimerosal during infancy and early childhood declined.

As this LA Times story shows, many autism advocates still aren’t convinced. I’m not surprised. I suspect that the emotional investment in thimerosal as a cause of autism, especially in the continuing absence of any other convincing theories, will keep many parents from accepting these results, and whenever I read these stories my heart almost breaks for them. They desperately want to know what caused their children’s autism, and without thimerosal they’ve got nothing. They just don’t know.

But that’s where we are: we just don’t know, and thimerosal isn’t the answer. It’s time to accept this and move on.

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