Thimerosal is a mercury-containing compound that has been used since the 1930s as a preservative in vaccines. Why was thimerosal introduced into vaccines? Well, early vaccines were administered from multi-dose bottles, in which bacteria could grow. In one particularly disastrous incident in 1928, 12 children in Australia died from staph infections after getting the diptheria vaccine from the same multi-dose bottle. After the introduction of thimerosal, bacterial infections caused by vaccination virtually disappeared.
Fast-forward 70 years, to the modern anti-vaccination movement. Following the late 1990s, a small number of activists, led in more recent years by J.B. Handley (who founded Generation Rescue in 2005) and a few others, decided that the mercury in vaccines causes autism. Robert F. Kennedy Jr. wrote articles promoting his crackpot notion of a large government conspiracy to cover up the harm being caused by thimerosal. The movement took off, especially after former Playboy model Jenny McCarthy became the “face” of Generation Rescue.
Was there every any scientific support for the link between thimerosal and autism? From the late 1990s to the present, scientists have looked closely at the evidence, and every well-done study has pointed to the same conclusion: thimerosal in vaccines has no link to autism. In one very large Danish study, autism rates rose after thimerosal was removed from vaccines. Another study looking at California, Sweden, and Denmark found the same thing. These results directly contradict the claim that thimerosal causes autism.
Despite the lack of evidence, the anti-vaxers have continued to wage their war against vaccines on two fronts. Last month, they lost the final battle in one effort, which claimed that the measles-mumps-rubella (MMR) vaccine causes autism. That battle started with the now-discredited 1998 study published in The Lancet by Andrew Wakefield. After the British General Medical Council ruled that Wakefield acted “dishonestly and irresponsibly”, the Lancet formally retracted the original paper, and a few days later Wakefield was forced to resign from the institute he founded in the U.S. to promote his claims.
Thimerosal was the “second front” in the anti-vax war, and now they’ve lost this one too. Last Friday, a special vaccine court ruled on three cases in which parents were suing on behalf of their autistic children. In each case, the parents claimed that thimerosal had caused their child’s autism. In each case, the Special Master (a judge) ruled definitively against the parents. The result was a slam-dunk win for science.
The three rulings take up over 600 pages, far too much to summarize, so I’ll just excerpt briefly from two of the conclusions. Special Master Denise Vowell, in the Dwyer case, issued a particularly devastating decision, ruling that claims about mercury were completely implausible and that the parents’ notion of “regressive autism” had no basis in science:
Petitioners propose effects from mercury … that do not resemble mercury’s known effects on the brain, either behaviorally or at the cellular level. To prevail, they must show that the exquisitely small amounts of mercury in TCVs [thimerosal-containing vaccines] that reach the brain can produce devastating effects that far larger amounts experienced prenatally or postnatally from other sources do not. … In an effort to render irrelevant the numerous epidemiological studies of ASD [autism spectrum disorder] and TCVs that show no connection between the two, they contend that their children have a form of ASD involving regression that differs from all other forms biologically and behaviorally. World-class experts in the field testified that the distinctions they drew between forms of ASD were artificial, and that they had never heard of the “clearly regressive” form of autism about which petitioners’ epidemiologist testified. Finally, the causal mechanism petitioners proposed would produce, not ASD, but neuronal death, and eventually patient death as well. The witnesses setting forth this improbable sequence of cause and effect were outclassed in every respect by the impressive assembly of true experts in their respective fields who testified on behalf of respondent.It’s interesting that Vowell found that even if the “exquisitely small” amounts of mercury in vaccines had an effect, they wouldn’t cause autism. It was also somewhat sad to see how a well-known statistician, UCLA professor Sander Greenland, appearing in support of the thimerosal-autism link, embarrassed himself by presenting testimony that “largely represented an opinion based on a set of assumptions,” according to the ruling. Greenland’s arguments relied entirely on the existence of “clearly regressive autism,” but the Special Master pointed out that Greenland “was not qualified to opine on its existence.” Ouch.
And here is an excerpt from the 122-page decision of Special Master George Hastings in the King case:
...the evidence is overwhelmingly contrary to the petitioners’ contentions. The expert witnesses presented by the respondent were far better qualified, far more experienced, and far more persuasive than the petitioners’ experts, concerning the key points. The numerous medical studies concerning the issue of whether thimerosal causes autism, performed by medical scientists worldwide, have come down strongly against the petitioners’ contentions. Considering all of the evidence, I find that the petitioners have failed to demonstrate that thimerosal-containing vaccines can contribute to the causation of autism.”I’m not optimistic that these clear, decisive rulings will have any effect on the conspiracy theorists in the anti-vax movement. Indeed, over at Age of Autism, they’ve already posted an article titled “Special Masters Protect Vaccine Program and Deny Justice to Vaccine-Injured Children.” The article, which is a combination of denialism and conspiracy mongering, claims that the trials ignored the science in order to defend the government’s vaccine program.
On the contrary, scientists studying autism want nothing more than to understand its cause and eventually to produce effective treatments. A growing body of evidence points to genetic factors behind ASD, but it will take much more work to pin down the complex combinations of genes that cause the various behaviors now called autism spectrum disorder. For example, a recent (2007) study by Sebat et al. found a clear link between ASD and de novo copy number variation (de novo mutations are those that arise for the first time in the children). We need more studies like this one if we’re to figure out this disease.
After the ruling, Alison Singer of the Autism Science Foundation said “It's time to move forward and look for the real causes of autism.” Well said. I hope that some in the anti-vax movement will recognize that if they truly want to find a cure for autism, they will support the science instead of insisting, as they do now, that more effort be poured into research on discredited hypotheses. The thimerosal-autism hypothesis is dead.