Thimerosal and autism – an overview, Part I

Ever since I wrote about the MMR/autism “controversy” a few months ago, I’ve been meaning to visit the subject of thimerosal in vaccines. I’ve been reluctant to take this on as it is a very large, complex subject, and a couple of blogposts won’t even scratch the surface. I’ll attempt to give the highlights of the controversy and debunk some of the anti-vaccine myths and conspiracy theories that have sprung up around the issue, but for those interested in a deeper look, there will be links galore at the end of the series for further reading. If you have an hour or ten, try to get to them.

What is thimerosal? Thimerosal (sometimes misspelled thimerisol) is an organomercury compound, meaning it contains both atoms of carbon and mercury, which was used in the cosmetic and medical industries since the late 1920s. Thimerosal is approximately 50% mercury by weight, the molecule contains an ethyl mercury group, and is in fact degraded into ethyl mercury in the body.

Why is it in vaccines? Thimerosal was originally used in vaccine vials in the early 1930s as a preservative, because of its ability to prevent bacterial and fungal contamination of vaccine vials. It was never used in live-virus vaccines (such as MMR or chickenpox), as this would have inactivated the vaccine viruses and rendered the vaccines useless. As of 1999, thimerosal has been phased out of vaccines in the US; most vaccines available were thimerosal-free by early 2002 (the CDC examined over 400 pediatricians’ clinics in Feb 2002 and found only 1.9% of the vaccines contained thimerosal; the last batches were to expire in November of that year). The only vaccines that still contain thimerosal in the pediatric population are some of the flu vaccines and the diphtheria-tetanus one (the latter is usually given to older children). See here.

How was the thimerosal/autism connection first made? After Wakefield’s MMR/autism brouhaha took off, other lobbying groups made up of parents convinced vaccines caused their children’s autism decided to look into other vaccines.
In 2001, a group of parents, among them Sallie Bernard and Lyn Redwood, mothers of autistic children and the founders of the lobbying group Safeminds, published a paper in a non-peer reviewed journal, Medical Hypotheses. They claimed there were similarities between autism and mercury poisoning, and hypothesized that autism is, in effect, thimerosal-induced mercury poisoning as a result of vaccination. These claims have been debunked by Bauman et al in their 2003 paper.

How do we know mercury is toxic, and what does this have to do with thimerosal? Why were vaccines a particular concern? Mercury has been known to be toxic for millennia; mercury compounds used to be common in cosmetics and pigments, and milliners who made felt hats were especially prone to poisoning – the symptoms of which are many and can even mimic other diseases. Mercury is toxic to humans in its elemental form (in gas much more than in liquid; breaking a thermometer should not be cause for alarm), as mercury salts, or as organomercury compounds, such as thimerosal.

As thimerosal is a known neurotoxin (in doses much larger than given in vaccines, and as derived from animal studies) and infants were, over the years, being given more vaccines, there was concern that the cumulative dose of mercury given in vaccines to children under 6 months old would exceed safety levels. In fact, the dose given in the 1990s did exceed the level deemed safe by the EPA (though not the higher levels deemed safe by other regulatory agencies, such as the WHO, FDA or the ATDSR (Agency for Toxic Substance Disease Registry). As a safety measure,the EPA’s permissible levels were 1/10th of the amount of mercury known to cause harm; nevertheless, the guidelines were written according to what is known about other mercury compunds such as methyl mercury (present in the environment and in fish; most babies partake of it during breastfeeding), not thimerosal, which, to remind you, degrades into ethyl mercury.

As a result of the controversy, the pharmacokinetic properties of thimerosal-containing vaccines were studied on a group of infants; it has been shown by Pichichero et al that mercury in thimerosal is rapidly eliminated from infants’ blood via the gut and the kidneys.* (Note: this study does not prove conclusively, though it does suggest, that ethyl mercury is less toxic than methyl mercury and the safe limits should be judged according ly. We will be discussing this regarding animal experiments in the next installment).

Why was thimerosal phased out from most vaccines in the US between 1999-2002? Wasn’t that an admission of guilt by the CDC/FDA/The powers that be? The vaccine was removed not because, as the conspiracy theorists allege, the government “knew” that thimerosal in vaccines was causing autism or any other disease, but in accordance with the precautionary principle, it seemed like a good idea to remove the thimerosal in response to public concern and to prevent mass anti-vaccination hysteria. Thimerosal in vaccines was also becoming increasingly unnecessary in affluent countries as single-vial doses of vaccines became the vogue, which were less likely to be contaminated and thus needed no preservative. In any case, the FDA had already banned thimerosal in OTC products, such as contact lens solutions, the year earlier (which, unlike vaccines, can be used in an unregulated fashion) because it was deemed correct to minimize exposure to mercury compounds as much as possible. In other countries, thimerosal was removed even earlier.

As we know, the conspiracy theory crazies had an absolute ball with this, intimating “they” knew all along they were injecting “toxins” into little babies and removed it only due to the efforts of the “gallant mercury warriors”. I sincerely hope the FDA has taken note, and doesn’t blanket remove any other “suspect chemicals” without sufficient evidence of harm.

Is there any evidence that thimerosal is responsible for the ‘autism epidemic’? No, nor is there any reliable evidence of an “autism epidemic” at all. While it’s true that the apparent incidence of autism has risen meteorically (from 1/10000 in 1970 to about 1/150 today), there is strong evidence that the cause is better diagnosis of autism due to the greater alertness of the public, including the medical community, to the syndrome, and also the broadening of the diagnostic criteria of autism-spectrum disorder. Meaning that in the past, children with severe autism were much more likely to be labelled mentally retarded, whereas high-functioning autistics (such as those with Asperger’s Syndrome) were merely considered odd, but normal.

Either way, if thimerosal in vaccines was indeed causing oodles of autism in children, once they were removed, one would expect a sharp drop in new autism diagnoses in young children who’d never received thimerosal in their vaccines. Even the anti-vaccine activists agreed this is what would happen.

What happened to the autism rates once thimerosal was removed from vaccines? Absolutely nothing. Zip. Zilch. Nada.

They kept on climbing exactly as before. In large-scale, epidemiological studies from Denmark (twice!), the UK, and the USA (this last by the Vaccine Safety Datalink). The only “researchers” (and I use that term loosely) who were finding any sort of association between vaccines and autism, were Mark and David Geier, whom we’ll meet in more detail in the next installment. Their studies, to put it politely, had so many fatal flaws as to be worthless. (Put less politely, they were some of the worst, most manipulative pieces sh*t to be published. Sort of like their authors, mind). We’ll be going over why in the next part.

Another attempt to claim that autism rates were dropping was attempted in 2005 in California by Rick Rollens, co-founder of the MIND institute, UC Davis, California. He declared this on the basis of a small dip in new total autism cases (of all ages) listed by the California Dept. of Developmental Services early in the year. However, his use of their database was incorrect, and in any case, a recent study has since debunked this.

In late 2007, another Vaccine Safety datalink study involving over 1000 children immunized with thimerosal-containing vaccines who were followed for 7-10 years was published in the New England Journal of Medicine, one of the world’s most prestigious medical journals. Both positive and negative associations with thimerosal doses were found, which essentially cancelled out each other and demonstrated, yet again, the lack of evidence of harm from vaccines. Of particular interest is that Sallie Bernard (the anti-mercury mom who co-founded Safeminds and first hypothesized about the link between vaccines and autism) was invited to be a constulant in the study’s design; when the study didn’t produce the results she’d expected, she flounced off and denounced the study she herself helped design. More on this at the always-excellent orac.

By 2004, the Institute of Medicine declared:

The committee also concludes that the body of epidemiological evidence favors rejection of a causal relationship between thimerosal-containing vaccines and autism. The committee further finds that potential biological mechanisms for vaccine-induced autism that have been generated to date are theoretical only.

This after in their previous (2001) review, they conceded that there was some biological plausibility to the theory, and the available evidence was insufficient to either reject or accept the hypothesis.

Does all this conclusively prove that thimerosal (or anything else) in vaccines never causes autism?
Not exactly. As I said in my previous post, you can’t conclusively prove a negative, and large-scale epidemiological studies can’t rule out a rare case of a child turning autistic as the result of a vaccine. However, this does mean that if this did happen, it would be only in a tiny minority of autistic children, and absent a plausible connection to mercury, the autism trigger could just as easily have been something else – a viral illness, a peanut butter sandwich, or even the birth of a sibling.

In the next installment, we’ll be looking at various issues and conspiracy theories pushed by the mercury militia and their enablers.

*Thimerosal is still being used in vaccines in developing countries, where preservatives are necessary due to transportation and refrigeration issues; this study was done in Argentina.

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6 Responses

  1. Thank you for this post. It was much more detailed than other articles I’ve read on this subject yet also clear and easy to read.

    There are a lot of anti-vax people in my community, so it’s good for me to be as knowledgeable on this topic as possible.

    BTW, my pediatrician won’t treat children who aren’t vaccinated. Do you have the same policy?

  2. Alisa, Thanks 🙂

    I work in a socialized medical system, and don’t have the option of shrugging off patients I don’t like, or those whose health beliefs I find unsavory. I have a few families who don’t vaccinate, and have spent a good deal of time talking myself blue in the face to change their minds. I’ve succeeded exactly once so far.

  3. Your series of posts are a great read for medical professionals. We recently wrote an article on a similar issue at Brain Blogger.

    The days of patients blindly following doctor’s directions are over. After all, how many times do we hear that we are ultimately in charge of our own health? When it comes to parents who choose not to vaccinate their children, a study reports two types: radical (refuses everything) and reformist (selectively opt out). Especially for the reformist group, pediatricians must be able to discuss anti-vaccination issues with the parent without acting like he/she is dumb, naive, or insane. In order to convert a parent from “reformist” viewpoints, acknowledging his/her questions and concerns as valid is the first step.

    We would like to read your comments on our article at our site. Thank you.


  4. Wow. I didn’t know your place had socilized medicine. However, being an American I am painfully ethnocentric and unaware of the other countries of the world. Except how nice it is to birth in Scandivania…J/K….

  5. Just kidding about Scandinavia….Not kidding about not knowing Jack about many other countries.

  6. Shaheen,

    I actually had hoped my posts would be more accessible to laypeople, but thanks.

    I love your website – we family physicians are all about the biopsychosocial model!

    Pinky – it’s not exactly like Canada’s or the UK’s but yes, medicine in Israel is largely socialized.

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