Thimerosal isn’t mercury. But to RFK Jr, facts don’t matter.
RFK Jr. just made vaccine disinformation federal policy. People will die from preventable illness—and that’s the point.
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A decision last month by RFK Jr.’s fake vaccine advisory panel is about to kill children—just not American ones.
The U.S. Health and Human Services Department (HHS), now under RFK Jr., has officially declared thimerosal (a safe vaccine preservative) is dangerous. It’s not.
But that lie, now policy, will ripple across the globe, destroying vaccine trust, access, and uptake in low-income countries.
In June, RFK Jr’s hand-selected ACIP members met to discuss several vaccines that are used in the US. They also lied—a lot. Dozens of lies. That isn’t surprising, since none of them have relevant expertise to serve on CDC’s ACIP (Advisory Committee for Immunization Practices), and most of them have espoused anti-vaccine and anti-science rhetoric for a long time.
One of the those lies?
A long debunked claim that thimerosal (which they call mercury) is neurotoxic and should not be in any vaccines. The only member of RFK Jr’s 8-member ACIP (typically, ACIP should have over 15 members, historically 17) to break from the conspiracy line was Cody Meissner. He correctly stated that “There is no scientific evidence that thimerosal has caused a problem.” Credit to him for that. This ACIP composed of ideologues voted 5 to 1 to recommend against vaccines containing thimerosal.
This week, RFK Jr. announced HHS will adopt this recommendation as policy.
In the US, thimerosal is not used in vaccines aside from a small percentage of flu vaccines (~7% of total flu vaccines) that are manufactured in multi-dose vials.
Some of you may be thinking, so what, then? Why are scientists making a big deal about this if it’s not going to impact us much?
Well, because this is a much bigger issue—a two-fold one, really.
We are now in a reality where wholly unqualified people, many outright conspiracy theorists, serve as decision-makers for American health policy. Legitimizing objective falsehoods is an incredibly dangerous direction to take as a country.
When America’s advisory team on vaccines endorses false claims about a safe chemical, this impacts every other country. That’s because nearly all countries take cues from us — a country that leads in scientific and public health innovation (at least, we used to).
Lies about thimerosal will kill lots of children, primarily in developing and underserved regions.
But, they won’t be affluent white kids, so I guess RFK Jr and his MAHA buddies view those kids as expendable.
RFK Jr. isn’t just wrong—he’s invested in being wrong. He profits by creating fear where there is none.
Why is RFK Jr so committed to this? Well, because his lucrative anti-vaccine agenda relies on it. He will profit more the more the lies get entrenched. He published this absolute dumpster fire of a book in 2014, where he spreads lies, conspiracy theories, and creates outrage that targets scientific experts, regulatory agencies, and physicians. And yep, his buddy Mark Hyman has been going along for the ride too.
Another anti-science activist group who was heavily involved in the mercury-vaccine lie? The Environmental Working Group, who published their own report in 2004 called “Overloaded? New science, new insights about mercury and autism in children” where they spread the very same lies. EWG and Ken Cook have tried real hard to scrub this from their history and position themselves as some bastion for science, but they are not—and they never have been.
But now, with RFK Jr leading HHS and cementing these objective lies as Federal policy, there is an entire world of lawsuits and litigation he has his eyes on. That’s his career — being a slimy litigator targeting companies and chemicals with emotional-laden stories for profit. Make no mistake, that is very much part of the endgame.
RFK Jr cares about power, money, and control—and thinks nothing of people that will die as a result of his actions.
Thimerosal is not mercury. It’s not methylmercury, either.
The claim circulating that ‘vaccines contain toxic mercury’ relates to a compound called thimerosal as has persisted for nearly 30 years. It has plagued public perception and led to measurable declines in childhood vaccine rates.
Now, ACIP and HHS are spreading these claims as if they are factual. They’re not. Lyn Redwood (President Emerita of Children's Health Defense, RFK Jr’s anti-science activist organization) claimed during the ACIP meeting that:
Mercury is the third most toxic element on earth, only behind polonium and plutonium and has no physiological role in the human body. There is also evidence that thimerosal is not an effective preservative at vaccine levels.
Let’s get the obvious out of the way. Mercury and thimerosal are not one and the same. This is thimerosal:
Thimerosal is the common name for the chemical Ethyl(2-mercaptobenzoato-(2-)-O,S) mercurate(1-) sodium, with the chemical formula C9H9HgNaO2S.
Mercury is an elemental metal with chemical formula Hg. Yep, that’s it. A pure metal —one that is liquid at room temperature.
I’m sure you can see that C9H9HgNaO2S is not the same as Hg.
Claiming thimerosal is mercury is like saying water (formula H2O) is hydrogen gas.
Or that glucose (formula C6H12O6) is diamonds (made up only of C atoms).
Or that air, a mixture of gases, is nitrogen gas only.
Unfortunately, people who spread these lies don’t understand a fundamental chemistry concept: a chemical compound is NOT the same as its elemental parts. That’s why table salt, sodium chloride (NaCl), doesn’t explode when you put it in water. Sodium explodes when you put it in water, though!
If anti-vaccine activists applied their logic to table salt, they SHOULD be telling everyone it explodes in water. But they’re not, right? That’s because their refusal to acknowledge facts only applies to lies they profit off of.
A more detailed discussion on thimerosal, mercury, and chemistry can be found here:
Some mercury-containing compounds can be harmful. Thimerosal isn’t one of them.
Mercury, the element (Hg), is a liquid metal at room temperature. Like other elements, it can bond with other atoms to form entirely new compounds—each with its own properties. Remember Mercurochrome? It was a topical antiseptic developed in 1918. Its chemical name is dibromohydroxymercurifluorescein, (C20H8Br2HgNa2O6) and a useful example to illustrate how mercury can be part of a compound that behaves nothing like elemental mercury.
Other relevant mercury-containing compounds are methylmercury and ethylmercury. They sound similar but are chemically and biologically distinct.
Ethylmercury (C₂H₅Hg⁺) is a short-lived ion (a charged compound) that’s rapidly eliminated from the body.
Methylmercury (CH₃Hg⁺) is more persistent. It bioaccumulates, crosses the placenta and blood-brain barrier, and in high doses can cause neurological damage.
While they both contain carbons, hydrogens, and mercury, they are not the same. These differences, while you may think they look small, dramatically change their chemical properties.
When Lyn Redwood claims “mercury is the third most toxic element,” she’s talking about methylmercury—a compound, not an element. And it has nothing to do with thimerosal.
People can experience mercury poisoning from elemental mercury and methylmercury.
Elemental mercury evaporates into mercury vapor (gas), when inhaled, can lead to neurological damage, kidney damage, or death.
Methylmercury (MeHg) is an organic (in chemistry, not farming terms) mercury compound that is everywhere in our environment. Methylmercury forms naturally in the environment when bacteria metabolize mercury from burning of coal and fossil fuels or volcanic activity. It climbs the food chain—especially in large fish—because it’s not easily broken down or excreted. That’s why we’re warned to limit tuna, not vaccines.
Thimerosal isn’t mercury, and it does not cause mercury poisoning.
At bare minimum, ACIP members should understand basic chemistry. But here we are.
Ethylmercury is an entirely different mercury compound related to thimerosal metabolism
Ethylmercury does not behave anything like methylmercury.
It does not bioaccumulate. It does not cross the placenta or blood-brain barrier. It does not cause neurotoxicity or organ damage.
How does ethylmercury relate to thimerosal? Thimerosal chemically, is a salt: a compound formed through attraction of negative and positive ions that dissolves and breaks apart into those ions in water. Thimerosal dissociates into ethylmercury (C2H5Hg+) and thiosalicylate (C7H5O2S-), both of which are rapidly processed and excreted from our body. Ethylmercury is filtered through the blood, metabolized in the liver, and rapidly excreted in our feces.
Thimerosal is not mercury, nor is it toxic.
Thimerosal was developed in the 1920s, and contrary to Lyn Redwood’s wildly uninformed claims, it is a very effective antimicrobial.
That’s why it is used to prevent bacterial contamination in multi-dose vaccines. Thimerosal is so effective only a miniscule amount is needed. It can inhibit microbial growth at concentrations of 0.001% to 0.01% w/v (weight/volume). In a vaccine vial, this would equate to 50 micrograms (µg) of thimerosal (containing 25 µg mercury ion) per 0.5 mL vaccine dose.
Some context?
A 6 oz can of albacore tuna can contain 30–50 μg of methylmercury, the mercury compound that does bioaccumulate and can lead to neurotoxicity from chronic exposure. The EPA reference dose (RfD) for methylmercury is 0.1 μg/kg/day, so if you were 150 lbs (68 kg), that means that one can of tuna contains between 4.4 and 7.3 times more methylmercury than the reference threshold. And guess what? That reference dose doesn’t apply to ethylmercury because it does not accumulate in the body.
Do you freak out about eating a can of tuna fish? That is a far greater risk than thimerosal (or ethylmercury) ever could be.
Thimerosal was already removed from the majority of US vaccines because of disinformation and public outcry.
Guess who started that fear and misinformation? You got it: RFK Jr., Andrew Wakefield, politicians, and others who don’t understand chemistry. I wrote about this in more detail here, but a short recap:
In 1997, NJ Representative Frank Pallone introduced a bill to amend the FDA Modernization act in 1997, to require the FDA to compile a list of all products that contained intentionally added mercury compounds, regardless of the type of mercury-containing chemical. During their review in 1998, FDA’s CBER (Center for Biologics Evaluation and Research) flagged thimerosal as a vaccine ingredient.
You might wonder, why do we even need thimerosal as a preservative?
Vaccines are injected intramuscularly. We want to make sure they are sterile, right? The manufacturing process ensures this is the case, but sources of contamination arise when vials are unsealed. If a vial contains 4 doses of vaccine, whoever is administering vaccines is going into and out of the vial multiple times: opportunity to inadvertently introduce microbes in the environment (or on their bodies) into the vaccine. If it was then administered to someone, that could be potentially very dangerous. Thimerosal keeps vaccines safe from being contaminated.
Ethylmercury, the metabolite of thimerosal, had no EPA safety threshold (we know this is because it is rapidly cleared from the body). But since a threshold didn’t exist, FDA, CDC, and AAP (American Association for Pediatrics) met and decided to use the guidelines for methylmercury (that 0.1 μg/kg/day level I noted above).
Unfortunately, that wasn’t scientifically appropriate. These groups acknowledged this was overly cautious, but because the vaccine schedule collectively would include a dosage of ethylmercury that exceeded the methylmercury limit, the decision was made to shift to single-vial vaccines that would not require preservatives.
Neil Halsey MD (an adviser for AAP) recommended that they exercise caution against the miniscule potential for any subtle neurological effects by replacing vaccines containing thimerosal for ones that did not, since at the time, there were no longitudinal comparative studies in children who had received thimerosal-containing vaccines versus those who had not. Today, we have those studies. We know thimerosal does not impact health outcomes.
In 1997, the cautious decision should have underscored how serious our regulatory agencies take safety, but instead, media attention created fear and alarm among parents, taking root in the autism community. People interpreted this to mean that if thimerosal was being removed, it must be harmful (this is the same logic with companies removing ingredients today, like parabens, also preservatives with overwhelming safety)
This was all happening when Andrew Wakefield was spreading his lies about vaccines. And just after that, RFK Jr switched his anti-science legal activism from farming to vaccines—because targeting ‘chemicals’ that you claim harm kids is a lucrative business endeavor.
Thimerosal was removed from all vaccines in the US, except multi-dose influenza vaccines (which account for roughly 7% of all flu vaccines administered in the US).
It had very real immediate and fatal consequences even decades ago. The recommendation to halt all thimerosal-containing vaccines applied to the Hepatitis B vaccine, given at birth (more here). While the recommendation selectively applied to children born to non-infected mothers, ~10% of hospitals suspended all Hepatitis B vaccines because of thimerosal fears. During this time, a 3-month old infant born to an infected mother died from overwhelming hepatitis B infection.
And that brings us to the bigger issue of RFK Jr’s HHS solidifying these lies about thimerosal as policy.
RFK Jr’s lies about thimerosal as US government policy undermines global vaccine efforts, and will cost children their lives.
This is why you should care about the recommendation to remove thimerosal, even though it is relatively irrelevant in the US:
Many countries globally use multi-dose vaccines that contain thimerosal because they offer benefits, especially among underserved and low income countries.
I know MAHA and RFK Jr. don’t care about these people, but you should. You should also care about what happens when infectious diseases aren’t controlled by vaccines in one country—because contrary to discourse on social media, pathogens don’t adhere to country borders. That’s why we are dealing with record-breaking levels of measles here in the US at this very moment.
Multi-dose vaccines are more cost-effective to manufacture, about 2.5 times less costly per dose than single-dose vaccines.
Multi-dose vaccines require less cold storage volume per dose (about 50% less) which is significant in regions with inconsistent access to refrigeration.
Multi-dose vaccines reduce the cost of transport per vaccine dose—so in developing nations with infrastructure limitations, this also reduces barrier to access vaccines.
UNICEF can procure inactivated polio vaccine at $1.25 per dose for multi-dose vaccine compared to $2.80 per dose for single-dose vaccines. When you are talking about low income countries, that 124% difference in cost per dose is substantial.
Many vaccines contain thimerosal globally, especially in low- and middle-income countries.
These include:
Inactivated influenza vaccines
DTP (diphtheria, tetanus, pertussis) vaccines, used in many Gavi-supported countries as it is much lower cost than the DTaP vaccine
Tetanus toxoid (TT), DT, and Td vaccines
Multi-dose hepatitis B vaccines
Meningococcal vaccines, especially those used in Africa’s meningitis belt
By legitimizing the claim that thimerosal is harmful as official US government policy, RFK Jr is undermining the public’s trust in vaccines globally.
What do you think will happen? You already know, because it happened here in the 1990s. People will be afraid of vaccines that contain this safe and effective preservative.
If they are afraid of those vaccines, they will refuse to receive them for themselves and their children. If vaccine rates decline, that means more preventable illness and death. It also means increased spread of those preventable infectious illnesses that are transmitted person-to-person. Increased spread means more outbreaks, harder to control infections, increased demand on healthcare infrastructure, which will also spill over into people needing healthcare for unrelated medical needs. It also means increased chance of microbial mutation and evolution, potentially opening the door to new strains of pathogens.
Thimerosal has been studied and cross-examined extensively. There is no link between thimerosal and neurological disorders.
If you live in a country that offer thimerosal-containing vaccines, do not be concerned: your vaccines are safe and effective. The benefits of vaccination far outweigh even a hypothetical risk of miniscule exposure to thimerosal.
Vaccination saves between 3-5 million lives every year.
For decades, RFK Jr. and his MAHA allies have worked to unravel that progress. His latest move will accelerate the damage. We cannot allow blatant falsehoods to shape science, medicine, and public policy.
RFK Jr. is not just misinformed—he is a deliberate threat to global health. His personal crusade, now cemented in federal policy, is reckless, self-serving, and lethal.
Now, more than ever, we all must join in the fight for science.
Thank you for supporting evidence-based science communication. With outbreaks of preventable diseases, refusal of evidence-based medical interventions, propagation of pseudoscience by prominent public “personalities”, it’s needed now more than ever.
More science education, less disinformation.
- Andrea
ImmunoLogic is written by Dr. Andrea Love, PhD - immunologist and microbiologist. She works full-time in life sciences biotech and has had a lifelong passion for closing the science literacy gap and combating pseudoscience and health misinformation as far back as her childhood. This newsletter and her science communication on her social media pages are born from that passion. Follow on Instagram, Threads, Twitter, and Facebook, or support the newsletter by subscribing below:
You must feel like banging your head against the wall when you hear the insanity coming from these people. Thanks for not giving up, and for continuing to disseminate accurate scientific information. Whenever I read your articles, I create a “cliff notes version” that I share on social media for people who aren’t as nerdy as I am. (I read extensively, so try to use this habit to benefit others.)
https://www.sciencedirect.com/science/article/abs/pii/S1382668919301875