Thimerosal (aka thiomersal) is a vaccine preservative that contains mercury. Many people fear mercury because of the known poisonous properties of methylmercury, although that is not the form found in thimerosal – when it dissociates it releases ethylmercury. Due to this precautionary fear, thimerosal was removed from childhood vaccines in 2002.

Please see end of this post for details. Note – a methyl group is not a problematic element per se (or more problematic than an ethyl group). The effects of a compound always depend on the molecular context. Methylation of DNA and proteins is a part of normal and healthy cell regulation, for instance.

The difference in the fate of these two forms of mercury in our bodies is surprising, highlighting the shortcomings of in vitro (test-tube or petri-dish) studies as reliable indicators of effects on living organisms. From the review Toxicity of ethylmercury (and Thimerosal): a comparison with methylmercury:

In vitro studies comparing etHg with meHg demonstrate equivalent measured outcomes for cardiovascular, neural and immune cells. However, under in vivo conditions, evidence indicates a distinct toxicokinetic profile between meHg and etHg, favoring a shorter blood half-life, attendant compartment distribution and the elimination of etHg compared with meHg.

Differences between in vitro and in vivo effects can be profound, which is hardly a surprise for biologists, and the fact that small differences in molecular make-up can play a big role is rather commonplace in the world of chemistry. Put in the words of the Credible She Hulk:

In chemistry, compounds have different physical properties than the constituent elements of which they are comprised. For example, elemental sodium is explosive, and elemental chlorine is a poisonous gas, yet sodium chloride is just table salt.

Sometimes molecules have an identical composition, but look like mirror images of each other (akin to our right and left hands) – and they may have completely different effects. The drug thalidomide owed its teratogenicity to the different effects of its ‘right’ and ‘left’-sided versions. For some drugs only one stereoisomer (enantiomer) may be effective, and the other inert. Chemistry is fascinating and complex.

We could all benefit from learning more chemistry.

Did thimerosal in vaccines cause harm?

Most vaccines are now manufactured with other preservatives due to public concern owing to fear by association. Some flu shots still contain thimerosal. The removal of thimerosal was not based on evidence of harm, and even in retrospect, none has been found, see statement from the World Health Organisation:

The Global Advisory Committee on Vaccine Safety concludes that there is no evidence of toxicity in infants, children or adults exposed to thiomersal (containing ethyl mercury) in vaccines.

and U.S. Food and Drug Administration:

Under the FDA Modernization Act (FDAMA) of 1997, the FDA conducted a comprehensive review of the use of thimerosal in childhood vaccines. Conducted in 1999, this review found no evidence of harm from the use of thimerosal as a vaccine preservative, other than local hypersensitivity reactions (Ball et al. 2001).

Why is there methylmercury in fish? Is that a problem?

A natural bacterial methylation process converts inorganic mercury in the ocean into methylmercury. It is not readily eliminated and instead gets biomagnified in the food chain – from bacteria, to plankton, through the whole range of prey organisms all the way to the predatory fish at the top.

Methylmercury causes so much trouble because of its ability to bind with amino-acids. It fools the body (and its transport proteins) that it is a normal part of a protein. It binds strongly to proteins and much of it remains in the body indefinitely. Wikipedia summarises its interesting fate inside us:

The methylmercuric-cysteinyl complex is recognized by amino acid transporting proteins in the body as methionine, another essential amino acid.^[12] Because of this mimicry, it is transported freely throughout the body including across the blood–brain barrier and across the placenta, where it is absorbed by the developing fetus. Also for this reason as well as its strong binding to proteins, methylmercury is not readily eliminated. Methylmercury has a half-life in human blood of about 50 days.

Should you worry about mercury exposure from fish? The recommendations to be careful with tuna and other big predatory fish are first and foremost for pregnant women and children, but if others like to be on the safe side, they can follow them too: limit your intake to a couple of cans of tuna per month (3-9, depending on which kind). The limits should by no means make us scared of eating fish, because fish is also a great source of nutrition. In some areas, fish with high levels of the mineral selenium may also help mitigate the problem by effectively binding to methylmercury, though more research is needed to clarify the effects.

The fear of mercury may do harm in the developing world

People in the developing world are in dire need of well preserved vaccines, and the health care sector doesn’t necessarily have the resources to supply newer, different formulations to all who need them. An expert opinion on drug safety states:

…a large body of evidence from animal and epidemiological studies has accumulated on the safety of thiomersal. Ironically, these data have become largely irrelevant in wealthy countries, where mono-dose, thiomersal-free vaccines have been introduced as a precautionary measure in almost all childhood vaccines, in part related to residual public scepticism.

In poor countries, multi-dose vials remain important for vaccine delivery. There is a real danger that this controversy may result in the loss to the world of thiomersal as a preservative, simply from popular pressure. In reality, it would be impossible to cease overnight using thiomersal and maintain the supply of vital vaccines. This paper reviews and summarises the data available from published studies on mercury toxicity, and thiomersal in vaccines in particular, that overwhelmingly indicate continued use of thiomersal is safe in those countries where it is most needed.

What about autism?

Many scared parents blamed mercury in vaccines for autism, although no connection has been found after extensive research. You can read more about it in my piece The great myth of vaccines and autism. I also include a sample of studies here below.

Read more in The great myth of vaccines and autism

• “Association between thimerosal-containing vaccine and autism. Conclusion: The results do not support a causal relationship between childhood vaccination with thimerosal-containing vaccines and development of autistic-spectrum disorders.”
• “Continuing increases in autism reported to California’s developmental services system: mercury in retrograde.”
• “Thimerosal exposure in infants and developmental disorders: a retrospective cohort study in the United kingdom does not support a causal association.”
• “Immunizations and autism: a review of the literature. Our literature review found very few studies supporting this theory, with the overwhelming majority showing no causal association between the Measles-Mumps-Rubella vaccine and autism. The vaccine preservative thimerosal has alternatively been hypothesized to have a possible causal role in autism. Again, no convincing evidence was found to support this claim, nor for the use of chelation therapy in autism.”
• “In this article, we review the history of the identification and classification of autism and the origin of the now widely-debunked autism/vaccine hypothesis. […] Finally, the evidence that autism is fundamentally a genetic disease is discussed, including family studies, the role of DNA copy number variation and known single gene mutations.”

For a good site on autism, see also: Autism Science Foundation.

By 2002 mercury was removed from the routine vaccine schedule. Science based medicine wrote about the lack of effect on the rates of autism incidence, predicted by people who campaigned for thimerosal’s removal:

…rates have continued to rise without even the slightest change in the rate of increase. This is a powerful refutation of the thimerosal-autism hypothesis, and has been replicated in other countries.

Further, the best epidemiological evidence suggests that the rise in the diagnosis rate of ASD is an artifact of broadening the definition of autism, diagnostic substitution, and increased surveillance. Therefore there isn’t really an autism “epidemic” just a change in the definition and efforts to make the diagnosis.

Why do people still claim there is a connection?

Many people continue to spread the claim that vaccines cause autism. For distressed parents it may be important to find a culprit, and some people make a profit from selling a supposed cure based on removal of the purported cause.

Image by Refutations to Anti Vaccine Memes

As an example of continued autism-claims, you can read a post on a blog on learning disabilities and parenting called I Speak Of Dreams – they have taken a look at an anti-vaccine document with over 70 papers that allegedly prove a connection between autism and vaccines. This tactic is an attempt to overwhelm the reader by the sheer number of links and citations.

Update: this list, still often used by anti-vaccine proponents, has grown to include 124 papers, each and every one of which has been addressed in a very long and detailed blog post by a doctor here.

When you sit down and look at the articles in this list you find one by one that they a) don’t actually relate to the topic; or b) aren’t research but opinion pieces; or c) don’t actually conclude that which you were led to believe; or d) have vital methodological flaws; and e) have not been confirmed by further research.

I will give an example from this list that exemplifies quite a few of the points above and demonstrates the misunderstanding around mercury. In this study from 1994, presented to the reader in this context as evidence for vaccine-autism link, the researchers looked at reactive glial cells in the visual cortex of monkeys. They were looking at primate brains – that at least could be relevant as a piece of the puzzle for a body of research looking at what the effects could be on humans. But the type of mercury exposure they studied? It was methylmercury – the kind found in fish but not in vaccines.

Not only that, but the monkeys were exposed to very high doses: 50 micrograms methylmercury per kilogram body weight per day, for months. The equivalent of which would be that a 30 kg (66 lbs) monkey would be eating 30 cans of tuna per day. Anyone arguing for a connection between vaccines and autism should lose all credibility after putting this kind of study forward as relevant evidence.

Why? Because, once again, methylmercury bioaccumulates, ethylmercury does not. Methylmercury is known for its poisonous effects and would never be used in vaccines.

It’s important to understand that different substances can have very different effects in the body. The way to evaluate their effect is to look at the best scientific evidence available on the substance in question. If you hear something that sounds scary about vaccines, you should try to learn more about it from legitimate sources such as the World Health Organisation or other science and health care institutions. The natural world is nuanced and looking at it properly takes a lot of time and care.

If you’d like to know more about vaccine ingredients, you can also read my post Aluminum in perspective. Or to start with, here’s a great way to kindle anyone’s interest: take a look at some of the wonderful chemistry infographics by Compound Interest.

Infographic by Compound Interest, Creative Commons licence CC BY-NC-ND

References for the Mercury in retrogade -infographic

The statement from the World Health Organisation on thiomersal.

The Global Advisory Committee on Vaccine Safety concludes that there is no evidence of toxicity in infants, children or adults exposed to thiomersal (containing ethyl mercury) in vaccines.

Thimerosal in Vaccines, FDA table on vaccines with thimerosal:
Flu shots contain 1-25 µg ethylmercury – while there are other flu shots that contain none.

FDA report on Mercury Levels in Commercial Fish and Shellfish (1990-2010):
Tuna fresh/frozen all: 0.39 ppm. Different types of tuna vary greatly, from 0.1 to 0.7.

Calculated with 0.39 ppm mercury in tuna, which is 0.39 µg/g. A tuna slice is estimated to be about 15 g (or half an ounce). Four pieces = 60 grams, so

60 g * 0.39 µg/g = 23.4 µg     – equivalent to flu shot which has 1-25 µg.

The WHO tells us that:

Ethyl mercury is actively excreted through the gut, and has a half-life of less than a week.

And the CDC has the information on absorption:

Methylmercury is the form of mercury most easily absorbed through the gastrointestinal tract (about 95% absorbed).

Additional info on mercury toxicity:

Review study on mercury: Among the organic mercury compounds, MeHg is the most biologically available and toxic (Scheuhammer et al. 2007)