Vaccines don’t cause mercury toxicity; the mercury compound present in some vaccines is different from the one in seafood

Key takeaway

Multiple studies show that vaccines don’t cause mercury toxicity. Some vaccines contain a mercury compound called thimerosal, a preservative that prevents the growth of harmful microorganisms. This compound is chemically different from the form of mercury that accumulates in some seafood and is also not associated with neurodevelopmental problems in children.

Reviewed content

Verdict:

Misleading

Claim:

“Doctors: don't eat fish when you're pregnant to avoid mercury getting into your baby Also doctors: vaccines containing mercury are safe to inject into your baby”

Verdict detail

Incorrect: Vaccines don’t cause mercury toxicity. The mercury compound thimerosal was removed from most childhood vaccines over two decades ago.
Misleading: Thimerosal contains ethylmercury, which is chemically different from methylmercury, the form of mercury that accumulates in seafood. While methylmercury is neurotoxic at relatively low levels, no evidence suggests that ethylmercury is harmful in the small amounts used in vaccines.

See our method of evaluation

Full Claim

“Doctors: don't eat fish when you're pregnant to avoid mercury getting into your baby Also doctors: vaccines containing mercury are safe to inject into your baby”

In late January, a Threads post with over 3,000 interactions claimed that doctors’ advice regarding mercury exposure in infants is inconsistent. Specifically, the post argued that while doctors recommend that pregnant women not eat fish “to avoid mercury getting into [their babies]”, they consider mercury-containing vaccines to be safe for children.

The post likely refers to vaccines containing thimerosal (also known as thiomersal in countries other than the U.S.), a mercury compound used in some vaccines to prevent contamination with harmful microorganisms.

Because some chemical forms of mercury are neurotoxic at relatively low doses, anti-vaccine groups have repeatedly blamed thimerosal for neurological problems in children, particularly autism. This is a myth with no scientific basis. Multiple studies involving hundreds of thousands of children have shown that thimerosal-containing vaccines aren’t associated with an increased risk of developing autism, as Science Feedback explained on several occasions.

Yet, the belief that vaccines are neurotoxic persists, as the Threads post illustrates. But the post is misleading in several ways.

First, thimerosal was removed from most childhood vaccines more than two decades ago. Second, not all forms of mercury are the same, as chemist Nigel Brown (who goes by the online moniker NileRed) explained in this video. The mercury compound that accumulates in fish differs from that used in small amounts in some vaccines, which hasn’t been associated with harmful effects.

These two facts make current concerns about mercury toxicity from childhood vaccines unjustified. We explain in detail below.

The form of mercury present in some vaccines is chemically different from that accumulating in seafood and doesn’t cause toxicity

Mercury is an element that can exist in the environment in different chemical forms. In its elemental (metallic) form, mercury can be toxic to the nervous system, the kidneys, and the lungs, both through chronic exposure or acutely due to the inhalation of mercury vapor.

Certain types of bacteria can convert elemental mercury in the environment to an organic form of mercury called methylmercury, which is a potent neurotoxin^[1]. Human exposure to this form of mercury occurs primarily through the diet. Shellfish and certain species of big fish, such as tuna, swordfish, shark, and king mackerel, can accumulate high levels of methylmercury that enter the bodies of people who consume their meat.

Methylmercury exposure is particularly concerning during pregnancy and breastfeeding as it can negatively impact brain and nervous system development in the fetus and during infancy. For this reason, U.S. and European regulators recommend limiting the consumption of seafood species high in methylmercury to one to two servings per week in the general population, but particularly during pregnancy and breastfeeding.

It is false, however, that pregnant women should avoid fish altogether, as the post claimed. Consuming one to four seafood servings per week is associated with a lower risk of cardiovascular disease in adults and better neurodevelopmental outcomes in children compared to no seafood consumption^[2]. Therefore, rather than avoiding fish, it is recommended that pregnant women choose small fish species containing low levels of methylmercury instead.

Contrary to what the Threads post implied, vaccines contain neither elemental mercury nor methylmercury. Instead, some vaccines contain thimerosal, the brand name of a mercury compound used as a preservative in some vaccines and whose main ingredient is ethylmercury. Research suggests that this compound is broken down and cleared from the body more rapidly than methylmercury, which makes it less likely to accumulate and cause toxicity.

Despite persistent claims that thimerosal in vaccines causes neurodevelopmental problems in children, multiple studies involving hundreds of thousands of children have shown no association between thimerosal-containing vaccines and autism^[3-5] or other neuropsychological disorders^[6,7]. The World Health Organization and the European Medicines Agency also stated that no evidence suggests that the low amounts of thimerosal in vaccines cause harm.

Mercury is no longer present in most routine childhood vaccines

Thimerosal has been used as a preservative in many pharmaceutical products, including vaccines, since the 1930s. This chemical helps improve vaccine safety by preventing the growth of bacteria and fungi in multi-dose vaccines, which are more likely to suffer contamination because they require inserting a needle into the same vial several times.

In 2001, the U.S. Food and Drug Administration (FDA) removed thimerosal from routine childhood vaccines. Likewise, the European Union and other countries like the U.K. also removed thimerosal from most pediatric vaccines in the early 2000s. But this wasn’t because thimerosal in vaccines is toxic—a belief that lacks scientific support—but as a precautionary measure to reduce overall infant exposure to mercury.

This means that thimerosal is no longer present in childhood vaccines in the U.S., with the exception of certain multi-dose influenza vaccines. Even in these cases, single-dose, thimerosal-free alternatives are available upon request.

About the post’s author (mommavspharma, Kaylee Marie, Kaylee Winfield, Kaylee Boynton)

The post was published by the account @mommavspharma, which has roughly 20,000 followers on Threads and almost 30,000 on Instagram. The link in her Instagram bio directs users to a website that sells unproven “detox” supplements to supposedly remove parasites and heavy metals from the body. As Science Feedback explained before, such products lack scientific support, are unnecessary, and might even be harmful in some cases.

Her name on Threads and Instagram is listed as “Kaylee Marie”. However, her website promotes a book about the alleged dangers of vaccines identifying her as “Kaylee Winfield”. A search for that name led us to this X/Twitter account, which displays a profile picture of a woman resembling the one in the Threads and Instagram profile picture. This suggests that the same user also goes by the name “Kaylee Boynton”.

Several of her posts on X also shared a link to her AllMyLinks profile. AllMyLinks is a social media management tool that enables users to share their content across multiple platforms. Her profile lists the website infernocannabishealth.com, with the business name “Remedy Holistic Wellness”. Multiple individuals left reviews about Remedy Holistic Wellness and another business allegedly owned by Boynton, claiming that she accepted payment and then failed to deliver on promised goods and services, alerting about a potential scam.

We reached out to Winfield for comment and will update the review if more information becomes available.

REFERENCES

• 1 – Lin et al. (2023) On the Origin and Evolution of Microbial Mercury Methylation. Genome Biology and Evolution.
• 2 – European Food Safety Authority (EFSA) (2014). Scientific Opinion on health benefits of seafood (fish and shellfish) consumption in relation to health risks associated with exposure to methylmercury. EFSA Journal.
• 3 – Hviid et al. (2019) Measles, Mumps, Rubella Vaccination and Autism: A Nationwide Cohort Study. Annals of Internal Medicine.
• 4 – Jain et al. (2015) Autism Occurrence by MMR Vaccine Status Among US Children With Older Siblings With and Without Autism. JAMA.
• 5 – Taylor et al. (2014) Vaccines are not associated with autism: An evidence-based meta-analysis of case-control and cohort studies. Vaccine.
• 6 – Thompson et al. (2007) Early Thimerosal Exposure and Neuropsychological Outcomes at 7 to 10 Years. New England Journal of Medicine.
• 7 – Andrews et al. (2004) Thimerosal Exposure in Infants and Developmental Disorders: A Retrospective Cohort Study in the United Kingdom Does Not Support a Causal Association. Pediatrics.