Jan 08 2008

One More Nail in the Mercury-Autism Coffin

I have been following the story of claims that thimerosal in childhood vaccines causes autism for a few years, and writing about it often in this blog. Without putting too fine a point on it, there are two camps when it comes to the question of the relationship between thimerosal and autism. The first camp (of which I am a member) has looked at all the scientific evidence and has come to the conclusion that thimerosal does not cause nor contribute to autism, that vaccines in general do not cause autism, and that there is no autism epidemic (as is often claimed), but rather an increase in surveillance and expansion of the diagnosis. This camp includes almost the entire scientific and medical communities.

The other camp appears to be motivated by ideology, not science. They cherry pick and misinterpret the scientific evidence systematically to support their beliefs, but the science is against them. They are comprised of misguided journalists, ignorant celebrities, well-meaning but scientifically naive parents, dedicated ideological groups including those who are anti-vaccine, conspiracy-mongering activists, and a few incompetent scientists.

Three years ago, when I first started looking into this controversy in earnest, it was already quite clear that there was no link between thimerosal and autism. But there was still a little wiggle room for doubt, and one could argue that the definitive evidence was still yet to come. Science is about making predictions – and so both sides made theirs. Unfortunately, the anti-vaccine crowd are ideologues, not honest scientists, so when their predictions started proving false they began to rationalize and dismiss.

The clearest example of this was the prediction that if thimerosal in vaccines were contributing to the incidence of autism, then once thimerosal was removed from vaccines the incidence of autism should decline. I and other have pointed out already that thimerosal was almost completely removed from the routine childhood vaccine schedule by 2002 and yet we have not seen any decrease in autism rates. This replicates the experience of other coutries, like Japan, that have removed thimerosal without any subsequent decrease in autism. Here we are six years after the removal of thimerosal in the US – and now we have the definitive data on the effects of this removal on the incidence of autism: none.

The predictions of the scientific community have been validated. The predictions of the “thimerosal causes autism” crowd have been repudiated. This is a victory-dance-in-the-endzone moment (not the first, and probably not the last).

A new study, just published (Schechter R and JK Grether (2008). Continuing Increases in Autism Reported to California’s Developmental Services System. Arch. Gen. Psychiatry 65: 19-24.) looks at the incidents of reported autism in the California system, and shows no decreased in autism following the removal of thimerosal Рnot a blip. This is especially sweet as this is the same set of data that has been used for years by the other side to support their claims for a link.

David Gorski, writing for our new blog Science-Based Medicine, does a thorough job of reviewing this study and its place in the debate. I want to add to his excellent summary by discussing some of the reaction to this study.

Interestingly, the anti-vaccine crowd has responded to the failure of their predictions by (prior to this latest study) moving the goalpost. David Kirby, who predicted a decline by 2007, pushed back the date to 2011. Others have moved the goalpost back from predicting a decline in autism to a decline in the percentage rate of increase. Barbara Fisher, and anti-vaccinationist, touts this as evidence for a role of thimerosal in autism. Autism rates are still increasing, but the percentage rate of increase is slowing. But as I pointed out over two years ago, as the rate of autism diagnosis approaches the true rate of autism, the rate of increase should slow down and eventually the rate of new diagnoses should level off. We are not even at the leveling off period, the rate of increase has not even decreased, just the percentage rate – which is a statistical consequence of the absolute rate increasing.

Worse, Fisher says that this only means that it is the vaccines themselves, and not just the thimerosal, that must be to blame. No evidence is enough for the anti-vaccine believers.

In response to the latest study Mark Blaxill, who has promoted the thimerosal and autism hypothesis, echoes Fisher in saying that this just calls into suspicion the entire vaccine program. Blaxill, in fact, throws out every kind of lame spin he can conjure up – that this does not mean individual cases of autism were not caused by thimerosal, for example.

I am still waiting for the response from Kirby and others, but in the past Kirby has argued that the autism rates in CA have not gone down because of other environmental exposures to mercury. This claim, in addition to being highly implausible – that other environmental mercury exposures would perfectly compensate for the decrease in thimerosal – doesn’t pass even cursory investigation.

For example, according to this recent article:

In 2005, approximately 500 electricity-generating power plants emitted 48.3 tons of mercury, an increase of about 1 percent since 2000, according to a USA Today analysis of the EPA’s Toxics Release Inventory. Mercury emissions from all other industrial sources were down 33 percent collectively in the same period.


U.S. mercury emissions have been cut nearly in half since 1990 as municipal, medical and hazardous waste incinerators closed or installed modern pollution controls.

So if anything the total environmental burden of mercury has decreased significantly over this period. The question of “how much mercury is in the environment and human exposure” is a complex one. There are different types of mercury and routes of exposure, etc. But Kirby and other apologists who will not let go of the thimerosal/autism hypothesis have somewhat casually blamed environmental mercury for the failure of their hypothesis to accurately predict the continued rise of autism rates in the face of successful efforts to decrease environmental exposure to mercury.

So other sources of mercury does not save the hypothesis that thimerosal causes autism, and simply blaming vaccines themselves does not rescue this failed hypothesis. What these arguments show is that those who were trumpeting the false claim that thimerosal causes autism are anti-vaccine ideologues, largely unmoved by scientific evidence.

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