Vaccines don't contain 'harmful mercury'
Vaccines don't contain 'harmful mercury'
Misinformation around vaccines and thimerosal has persisted for decades
Have you ever heard the claim that vaccines contain mercury and that was a reason to have concern?
If so, you’re not alone. This claim relates to a specific chemical that contains mercury, called thimerosal. Unfortunately, like many vaccine myths, this persistent falsehood has plagued public perception and opinion, and has led to measurable impacts on rates of childhood vaccines.
This topic is going to start with some basic chemistry: there are key differences between an element and a chemical compound.
(I discussed this concept previously, including in my article on fluoride)
A chemical element is a pure substance consisting only of that atomic species. Unlike chemical compounds, chemical elements cannot be broken down into simpler substances by any chemical reaction. Chemical compounds are composed of several different elements and are arranged in different methods to bond these elements together. Examples include covalent and ionic bonds.
Chemical compounds are not the sum of their elemental parts. The identity, nature, behavior, and metabolism of these chemicals change when they are in compound form.
Table salt is a great illustration of this.
Table salt is an ionic compound composed of sodium ions (Na+) and chlorine ions (Cl-). Together they make sodium chloride, formula NaCl, a white crystalline solid that dissolves in water that we use to impart flavor to foods.
Elemental sodium is a metal at room temperature, and explodes when placed into water. Elemental chlorine exists as a yellowish gas at room temperature, and can be extremely toxic with low exposures.
However, when these two elements are combined, they make a completely different substance with properties distinctly different from their subunits: sodium chloride, what you know as table salt. This chemical imparts flavor to foods and provides important ions that our bodies use for essential physiological processes.
The beauty of chemistry!
Let’s apply this logic to claims related to ‘harmful mercury’ in vaccines.
Mercury is a metal element, one of the first identified metals. Mercury is designated by symbol (Hg), from the Greek hydrargyrum, meaning "liquid silver," or "quick silver". Mercury is indeed a liquid metal a room temperature and it was used experimentally in ancient civilizations.
These ranged from treating syphilis, as a pigment, to placating evil spirits. In more recent culture, you may be familiar with mercury thermometers. Mercury expands uniformly, making it a great candidate for thermometer liquid, but the toxicity of elemental mercury means it is almost entirely phased out from use in medical thermometers.
Just like sodium, mercury is found in other chemical compounds. Two we will discuss are ethylmercury and methylmercury.
Ethylmercury is an ion composed of an ethyl group (CH3CH2) attached to a mercury ion, giving it a chemical formula C2H5Hg+.
Methylmercury is an ion composed of a methyl group (CH3) attached to a mercury ion, giving it a chemical formula CH3Hg+.
While they both contain carbons, hydrogens, and mercury, you can see they are not the same. These differences, while they look small, actually change their chemical properties dramatically.
Certain types of mercury-containing compounds can be harmful and lead to mercury poisoning.
There are two main types of mercury poisoning:
Elemental mercury poisoning can occur via inhaling elemental mercury gas (vapor) or liquid mercury. This can cause a variety of side effects and long term neurological damage, kidney damage, or even death. This poisoning happens through exposure to elemental, or liquid, mercury metal.
The second type of mercury poisoning is caused by methylmercury exposure. Methylmercury (MeHg) is an organic mercury compound created by bacteria through methylation of environmental mercury from volcanic eruptions, burning of fossil fuels and coal, and other industrial processes. Methylmercury is found in the environment ubiquitously; almost everyone has a small amount in their bodies.
Methylmercury does bioaccumulate and is not readily metabolized and excreted by our bodies, so methylmercury poisoning can be dose-dependent. Methylmercury has a long half life (~60 days in the brain, 20 days in the blood), so multiple exposures over time can lead to potentially dangerous levels of MeHg in the body. This is more dangerous for fetuses as MeHg crosses the placenta readily, and during fetal development exposure to high levels of MeHg increases the risk for neurological and developmental disorders.
Methylmercury is also subject to biomagnification: small amount of methylmercury in the environment becomes magnified through the food chain via consumption of large amounts of organisms that contain small amounts of mercury.
This is why people are cautioned to limit consumption of large fish: since they prey on smaller fish which prey on other organisms/plants in the environment, the levels of methylmercury will be highest the further you climb on the food web.
Neither of these types of mercury are related to thimerosal: a chemical that is metabolized into ETHYLmercury.
Ethylmercury: another mercury compound plagued with controversy.
Ethylmercury, like methylmercury, is also an organic mercury compound, but that difference in number of carbons and hydrogens means it does not behave like methylmercury.
We encounter ethylmercury as a byproduct of thimerosal. Thimerosal is a thiosalicylate salt of ethylmercury developed in the 1920s, and it’s a very effective antimicrobial. It is used to prevent bacterial contamination in multi-dose vaccine vials.
Thimerosal can also be used in vaccine manufacturing to prevent contamination. Thimerosal is effective at inhibiting microbial contamination at a low concentration: 0.001% to 0.01%. In a vaccine vial, this would equate to 50 µg of thimerosal (containing 25 µg mercury ion) per 0.5 mL dose.
Prior to the 1990s, maximum exposure during infant immunizations according to the CDC-recommended schedule during co-administration of Hepatitis B and Haemophilus influenzae vaccines would be 187.5 µg of thimerosal (which is ~ 50% Hg by weight). For reference, 187.5 µg is 0.000187 grams: a miniscule level.
Unfortunately, public outcry in the late 1990s as a result of misinterpretation of information led to undue negative attention about thimerosal. The myths about potential harms have persisted for over 25 years. As a result, thimerosal was removed from all vaccines in the US, except multi-dose influenza vaccines. This was not due to safety concerns, but chemophobia and misinformation.
Due to unfounded public concern and the availability of single-dose vaccine vials, thimerosal is no longer a component in nearly all childhood immunizations across the US. However, many countries still rely on multi-dose vials for cost-efficiency and these vaccines containing thimerosal are absolutely safe for use.
Here’s the thimerosal controversy:
Thimerosal had been used as a preservative for over 50 years when a new FDA Modernization bill came into effect, which required the FDA to compile a list of all products that contained intentionally added mercury compounds, regardless of the type of mercury-containing chemical.
This bill came into effect at the height of awareness of methylmercury toxicity following the Minimata disaster of 1968, the Iraq seed poisoning in 1978, and two conflicting longitudinal studies from the 1980s between the Seychelles and Faroes where the potential harms of environmental methylmercury were described. To be clear, these incidents all relate to methylmercury.
In 1999, when the FDA was reviewing the safety of the maximum exposure of 187.5 µg in infants, they consulted several resources and determined that no safety guidelines existed for ethylmercury; only for methylmercury. The FDA then held a meeting with the Center for Disease Control (CDC) and the American Association for Pediatrics (AAP). Since no guideline existed for ethylmercury, they used the guidelines for methylmercury as a stand-in. As we just discussed, despite the names being similar, these compounds are incredibly different.
Even in 1999, those making this decision acknowledged the uncertainty. However, that 187.5 µg exposure of ethylmercury (above) exceeded the existing exposure limit for methylmercury set by the Environmental Protection Agency (EPA), but not the limit by the FDA. Even though there was no evidence that thimerosal had caused any harm, the decision was made to shift to single-vial vaccines, championed largely by Neil Halsey, MD., a long-time vaccine advisor for the CDC and AAP. Halsey did not, and maintains that he does not, have any concern that the thimerosal in vaccines caused autism.
Since there were no longitudinal comparative studies in children who had received thimerosal-containing vaccines versus children who had not, he advised that they exercise caution against the potential for any subtle neurological effects by replacing vaccines containing thimerosal for ones that did not.
While this caution should have underscored the weight that our safety and regulatory agencies place on all of these factors, people had the opposite reaction. Media attention created fear and alarm among parents, and took root in the autism community. Many interpreted this decision to mean that if thimerosal was being removed, it must be harmful.
Of course, it didn’t help that this came on the heels of the fraudulent 1998 “study” by Andrew Wakefield, one of the most singularly harmful pieces of disinformation about vaccines. As such, in 1999, claims about vaccines and purported links to neurological issues (specifically, autism) were rampant (I discuss that story here). This timing added fuel to the anti-vaccine fire and further damaged public trust in vaccines.
It also had very real immediate and fatal consequences.
The recommendation to halt all thimerosal-containing vaccines included the Hepatitis B vaccine, given at birth. The recommendation actually advised only to pause Hepatitis B vaccination in newborns born to non-infected mothers only. However, due to fears of giving thimerosal-containing vaccines, about 10 percent of hospitals suspended Hepatitis B vaccines outright. Unfortunately, during this time, a 3-month old infant born to an infected mother died from overwhelming hepatitis B infection.
Hindsight is always 20/20, but especially in this case.
If standards for exposure had been reviewed and set specifically for ethylmercury in 1999, it is entirely possible that 1) thimerosal wouldn’t have been removed at all, or 2) a slow phasing out of thimerosal use may have been recommended. Instead, an immediate halt to administration of all thimerosal-containing vaccines was implemented. Unfortunately, we know now that he pharmacokinetics vary wildly between these two mercury-containing chemicals in question.
A single carbon atom is the difference between happy hour and six feet under.
Methylmercury, as mentioned, is primarily introduced into humans via consumption of fish or rice. Most people have heard somewhere that excessive consumption of larger fish should be limited. Of course it is understandable that concerns arose about the other mercury-containing substances in immunizations when first introduced.
Remember though, these compounds are distinctly different. While the ethyl- versus methyl- may seem like a small difference, it dramatically alters the pharmacokinetics (the way they are absorbed, distributed, metabolized, and eliminated).
For example, ethyl alcohol, also known as ethanol, is the legal, consumable (in moderate amounts) form of alcohol you would find in beer, wine, and spirits.
It has the chemical formula C2H5OH. As many of us are aware, excessive consumption causes drunkenness, dizziness, and potentially poisoning. Overconsumption can be lethal, but at a dose of 10,000 mg of pure ethanol per kg of bodyweight.
Methanol, CH3OH, on the other hand, is extremely toxic and should not be consumed, inhaled, or even contact the skin. As little as a tablespoon of Methanol causes blindness, dizziness, confusion, ataxia, seizure, coma, and even death. The toxic effects of methanol can be experience if ingested at 800 mg per kg of bodyweight.
Methanol is over 12-times more toxic than ethanol, even though those chemicals look similar on paper.
The variation in toxic effects can be attributed to the metabolites the human body generates when it is breaking down the compounds. As you can see, despite their names sounding so similar, they are clearly very, very different from one another. The differences in pharmacokinetics between ethyl and methyl compounds are relevant in the case of thimerosal (metabolized into ethylmercury) and methylmercury in food.
Methanol and ethanol metabolism:
Ethylmercury and methylmercury are not the same.
The decision to remove thimerosal from childhood vaccines was based on the premise that ethylmercury behaves the same way as methylmercury.
The supposition was hypothetical at the time, and has now been disproven thanks to recent confirmation that the pharmacokinetic behavior of the two compounds differs significantly. Most of the literature concurs that the difference in behavior within the body is likely due to the size difference of the compounds, making it more difficult for ethylmercury to cross the blood-brain barrier.
Ethylmercury has a short half life, 3-7 days. It is rapidly metabolized and excreted by the body, and does NOT bioaccumulate. Even in the brain, ethylmercury has a much shorter half-life (14 days) compared to methylmercury (58 days). Ethylmercury is further metabolized into inorganic mercury, which has even lower toxicity as a result of low water solubility. In contrast, most methylmercury persists as organic mercury. Due to these differences, it is clear that unique safety guidelines for ethylmercury would be beneficial.
Thimerosal has been studied and cross-examined at length for neurotoxic effects and researchers have failed to find a link between thimerosal and neurological disorders.
If you live in a country that offer thimerosal-containing vaccines, please do not be concerned: your vaccines are safe and effective. The benefits of vaccination far outweigh any hypothetical risk of a miniscule exposure to thimerosal found in vaccines.
Remember: vaccination saves between 3-5 million lives annually. Unfortunately, pervasive myths like these about harmful ingredients in vaccines are still causing harm today, even when data demonstrate the concerns are entirely unfounded.
As always, thanks for joining in the fight for science!
Thank you for supporting evidence-based science communication. With outbreaks of preventable diseases, refusal of evidence-based medical interventions, roadblocks to scientific progress that improve food and crop sustainability, it’s needed now more than ever.
Your local immunologist,
Andrea