The Fluoride of the Progressives

Robert F. Kennedy Jr. has been making the rounds of talk shows talking up the idea that autism is caused by thimerosal, a mercury compound formerly used in this country as a preservative in childhood vaccines. While some right-wingers are also upset about thimerosal, most of the concern about it comes from the left, which is peculiar in a way since suspicion about vaccination is traditionally a conservative preoccupation that reflects a programmatic unwillingness to bear even tiny personal risks for the general good. To be sure, Kennedy and Arianna Huffington and various folks at Salon and Rolling Stone claim to support vaccination and relate their attacks on thimerosal to an anti-corporate theme. Their narrative is all about how Big Pharma conspired with bunch of bought-off scientists to cover up the connection between mercury and autism. Of course if thimerosal really is a cause of autism, the motives of its critics don’t matter very much. What is the state of the evidence?

No official scientific agency in the United States and Europe has reported any link between thimerosal and autism or any other neurological problem, but a consensus developed that it should not be used in vaccines because of a general concern about mercury exposure. For this reason, thimerosal has not been used in vaccines in this country since 1999 while research into the possible effects of thimerosal has continued without turning up any very definitive results. As with many recent epidemological issues—the health effects of electro-magnetic fields, for example—the technical difficulties of defining the real risks are formidable, in part because the association between the agent and the effect is so weak. Nobody is claiming that flu shots are to autism what cigarettes are to lung cancer. Mercury is not good for the nervous system, but the toxicity of mercury depends upon its chemical form. Thimerosal delivers ethyl mercury to the body whereas environmental exposure to mercury largely takes the form of methyl mercury. The pharmacology of ethyl mercury remains to be elucidated. While methyl mercury tends to linger in the body, ethyl mercury is rapidly excreted in the feces. On the other hand, some babies may not be as capable of excreting ethyl mercury as others; and ethyl mercury from a vaccine injection comes as a single big dose while methyl mercury from food and water is ingested a little at a time. A bolus of mercury may produce a more severe effect than an incremental exposure.

The case against thimerosal also assumes that the incidence of autism has increased markedly in step with the frequency of childhood immunizations. This assumption is not obviously valid. You sometimes read that autism was unknown before the introduction of vaccines with thimerosal, but what happened in the forties was the introduction of the term autism, not the sudden appearance of withdrawn, affectless children. Many cases of what we would now call autism are described in the older medical literature, just as millions of people died from heart attacks over the years even though myocardial infarction was only defined in the 20th Century. Because the clinical definition of autism is so vague, it is perfectly possible that its increasing incidence reflects to some degree the tendency of doctors to make trendy diagnoses in borderline cases, an instance of that syndrome inflation or societal hypochondria familiar from the ADD saga. In this regard, I find it telling that follow up studies of children diagnosed as autistic in recent decades find that many of them go on to college and careers while the autistic kids of yesteryear are still banging their heads against a wall.

Because as a rule it is hard and expensive to figure things out, none of the questions about thimerosal are going to be answered quickly and anybody who issues conclusive statements about the matter is overstating his or her case. That said, I confess that I lost interest in the issue a couple of years ago because of a simple consideration. Thimerosal was removed from vaccines in Denmark in 1992 but the reported incidence of autism continued to rise sharply thereafter (Science, Vol 301, Issue 5639, 1454-1455, 12 September 2003). That finding doesn’t guarantee that the mercury in vaccines has no relationship whatsoever with autism, of course; but it certainly deflates the notion that thimerosal is the fundamental problem. The research also reminds us that a giant and crucial experiment is already underway in this country. If thimerosal really was the cause of a U.S. autism epidemic, the epidemic should be coming to an end shortly since vaccines no longer contain thimerosal. Since this vast if accidental test will much larger and more conclusive than any feasible planned study, I think it makes sense to reserve scarce research dollars to deal with other issues.

Note: for a rundown of information on thimerosal that includes links to sources on both sides of the issue, see this Wikipedia page.