Some people are very invested in the idea that thimerosal in vaccines causes autism. They have looked and looked, but have been unable to find enough credible evidence to convince the scientific community. Thimerosal was removed from US vaccines several years ago, and you might have thought that would end the debate. It didn’t. The spotlight has shifted to other countries that still use thimerosal-preserved vaccines, such as Peru.

Anti-vaccine activist David Kirby said,

If thimerosal is one day proven to be a contributing factor to autism, and if U.S. made vaccines containing the preservative are now being supplied the world over, the scope of this potential tragedy becomes unthinkable.

The anti-vaccine website Age of Autism accuses US policy of

[making]…Kirby’s nightmare suggestion a reality. U.S. vaccine manufacturers have continued to ship thimerosal containing vaccine formulations all over the world, in effect offering a defiant double standard of mercury risk for infants from rich countries as compared to poor countries.

They accuse the US of keeping its own children safe while callously endangering children in other countries. Actually, the decision to remove thimerosal from vaccines in the US was a bend-over-backwards-cautious political decision rather than one based on evidence of harm. The vaccines used in Peru are not evidence of any evil plot, much less a US plot. The World Health Organization (WHO) and its Latin American partner the Pan American Health Organization (PAHO) decided to retain thimerosal as a vaccine preservative because of economic considerations and because they did not find evidence that it was harmful.

The Age of Autism website is making much of a new Peruvian study in hamsters that allegedly supports the thimerosal/autism link. The abstract is available in English, but the full article is in Spanish. Critics demanded an English translation to allow for proper peer review, only to be told they were prejudiced against Spanish and that the report had been peer reviewed. One critic thought “the fact that there is no formal translation of this report into any language that I am aware of makes it obvious that the report is no good – otherwise the scientific community would be falling over themselves to translate it.” I don’t think that’s a valid inference, but anyway it would be nice to be able to read the whole paper, not just the English abstract.

No problem. I’m fluent in Spanish. I read it. The research was not in my field of expertise, but it looked quite reasonable as far as I could tell. I don’t think it really matters whether the study is flawed, because even if its results are impeccable I’m not impressed that they tell us anything that would link thimerosal to autism.

The abstract says

The administration of thimerosal in doses equivalent to vaccines content was associated with low corporal weight, low encephalon weight and smaller stature in postnatal hamsters. Neurotoxic effects were also produced at encephalic level, at hippocampus (regions CA1, CA3, DG), cerebral cortex and cerebellum (Purkinje cells and granuloses cells) with decrease in neuronal density, neuronal necrosis, axonal dismyelinization and gliosis.

A US study in 2004 showed that only certain strains of mice were susceptible to thimerosal, and the findings in those mice were very different from the hamster findings.

Autoimmune disease-sensitive SJL/J mice showed growth delay; reduced locomotion; exaggerated response to novelty; and densely packed, hyperchromic hippocampal neurons with altered glutamate receptors and transporters. Strains resistant to autoimmunity, C57BL/6J and BALB/cJ, were not susceptible. These findings implicate genetic influences and provide a model for investigating thimerosal-related neurotoxicity.

A similar US study on those SJL/J sensitive mice was published in 2008. It used a dose 10 times higher and found that

results do not indicate pervasive developmental neurotoxicity following vaccine-level thimerosal injections in SJL mice, and provide little if any support for the hypothesis that thimerosal exposure contributes to the etiology of neurodevelopmental disorders.

So of the 3 studies in small critters, one showed no effects at 10 times the dose, and the two that did show effects didn’t show the same effects. For instance, one showed a decrease in neuronal density, the other showed densely packed neurons. They did both show growth delays (weight and height), but that is not characteristic of autism; and the other hamster and mouse findings do not coincide and do not match known findings in autism.

Animal studies are always problematical. The effects on hamsters may be very different from the effects in humans. There is no adequate animal model of autism.

The Peruvian study is not listed on PubMed, which does list many journals in other languages and provides English abstracts. It was published in an in-house journal, a publication of the Peruvian medical school where the research was done. Their articles are reviewed by an in-house editorial board, not sent out for external peer review.

Studies in non-English speaking countries are statistically more likely to be positive (i.e., there is a greater chance that published reports are false positives). Scientific standards may not be as high, and negative results are less likely to be published. I don’t think Peru does a whole lot of medical research; this is the first study I have ever read from that country.

The review of the literature in the introduction to the study is biased. It misrepresents the state of the evidence for and against thimerosal. It cites a series of papers as evidence that thimerosal causes autism, but every one of them is by the discredited anti-vaccine activists Geier and Geier. The many credible studies showing no correlation are barely mentioned in passing. It seems obvious that the authors of this paper have an agenda: to show that thimerosal should be removed from vaccines in Peru.

They do not make it clear that the decision to remove thimerosal from American vaccines had nothing to do with any documented dangers, but was a precautionary measure to reduce total mercury burden from all sources “just in case.”

Their bias is also shown by their comments, “While it is true, it is very difficult to extrapolate these findings to other animal experimentation groups and over human beings, our results, as the multiple scientific evidence recently published about thimerosal, clearly indicates the toxic nature of this substance at the same dose and the same chronology as human immunizations; therefore we suggest the employment of alternative preservatives in vaccines, especially those intended to pregnant women, neonates and small children based in the prevention and precaution principles of all medical interventions.”

The best scientific studies usually end with a statement to the effect that “if” their findings are confirmed, it “might” mean thus and so. These folks seem to me to have gone beyond the science into recommending social policy. And they have not considered whether there might be any downside to demanding alternative preservatives.

If the thimerosal hypothesis were correct, the rate of autism would be rising faster in Peru than in the US. I don’t think anyone has tried to determine if that is true. Without that kind of data, conclusions based on animal experiments are nothing but speculation.

So what do we have? One study showing thimerosal is bad for hamsters in a way that doesn’t match the findings in autistic children. One study showing thimerosal is bad for one strain of susceptible mice, but in a different way that also doesn’t match the findings in autistic children. Another mouse study contradicting the first one, showing 10 times as much thimerosal is not bad for mice, even the most susceptible strains of mice. That sure doesn’t add up to much. We may have learned something about hamsters, but we haven’t learned anything about humans or autism.

I think the Peruvian experiment may have been a waste of hamsters. PETA wouldn’t approve. But then PETA wouldn’t approve of the Peruvian policy towards guinea pigs, either: they let them run free on the dirt floor of their kitchens, then snatch them up and cook them as needed. I tasted “cui” when I visited a village in the Andes – it was pretty good.

After reading this study, I couldn’t help but ask, “Cui bono?”
(That’s Latin for “who benefits?” and also for “Does the guinea pig taste good?”)

Posted by Harriet Hall

Harriet Hall, MD also known as The SkepDoc, is a retired family physician who writes about pseudoscience and questionable medical practices. She received her BA and MD from the University of Washington, did her internship in the Air Force (the second female ever to do so),  and was the first female graduate of the Air Force family practice residency at Eglin Air Force Base. During a long career as an Air Force physician, she held various positions from flight surgeon to DBMS (Director of Base Medical Services) and did everything from delivering babies to taking the controls of a B-52. She retired with the rank of Colonel.  In 2008 she published her memoirs, Women Aren't Supposed to Fly.