Thimerosal carries no health risks and is almost never used anyway. So why are anti-vaxxers obsessed with it?

Editor's note: This story doesn't reflect more recent developments from the Advisory Committee on Immunization Practices, which has now recommended against using flu vaccines containing thimerosal. Experts, including a lone member of the committee, have questioned the decision given the lack of risk posed by the preservative.

An expert committee that advises the Centers for Disease Control and Prevention on vaccines is meeting for the first time since Health Secretary Robert F. Kennedy Jr. abruptly replaced the committee's 17 members with eight hand-picked ones on June 11, 2025.

The committee, called the Advisory Committee on Immunization Practices, generally discusses and votes on recommendations for specific vaccines. For this meeting, taking place June 25-26, 2025, vaccines for COVID-19, human papillomavirus, influenza and other infectious diseases were on the schedule. According to an updated agenda, however, the committee is now also scheduled to hear a presentation on a chemical called thimerosal and to vote on proposed recommendations regarding its use in influenza vaccines.

Public health experts have raised concerns about the presentation, noting that anti-vaccine advocates continue to promote confusion regarding the purported health risks of thimerosal despite extensive research demonstrating its safety.

I'm a pharmacist and expert on drug information with 35 years of experience critically evaluating the safety and effectiveness of medications in clinical trials. No evidence supports the idea that thimerosal, used as a preservative in vaccines, is unsafe or carries any health risks.

What is thimerosal?

Thimerosal, also known as thiomersal, is a preservative that has been used in some drug products since the 1930s because it prevents contamination by killing microbes and preventing their growth.

In the human body, thimerosal is metabolized, or changed, to ethylmercury, an organic derivative of mercury. Studies in infants have shown that ethylmercury is quickly eliminated from the blood.

Ethylmercury is sometimes confused with methylmercury. Methylmercury is known to be toxic and is associated with many negative effects on brain development even at low exposure. Environmental researchers identified the neurotoxic effects of mercury in children in the 1970s, primarily resulting from exposure to methylmercury in fish. In the 1990s, the Environmental Protection Agency and the Food and Drug Administration established limits for maximum recommended exposure to methylmercury, especially for children, pregnant women and women of childbearing age.

Related: 'Vaccine rejection is as old as vaccines themselves': Science historian Thomas Levenson on the history of germ theory and its deniers

Do vaccines have mercury in them? - YouTube

Watch On

Why is thimerosal controversial?

Fears about the safety of thimerosal in vaccines spread for two reasons.

First, in 1998, a now discredited report was published in a major medical journal called The Lancet. In it, a British doctor named Andrew Wakefield described eight children who developed autism after receiving the MMR vaccine, which protects against measles, mumps and rubella. However, the patients were not compared with a control group that was vaccinated, so it was impossible to draw conclusions about the vaccine's effects. Also, the data report was later found to be falsified. And the MMR vaccine that children received in that report never contained thimerosal.

Second, the federal guidelines on exposure limits for the toxic substance methylmercury came out about the same time as the Wakefield study's publication. During that period, autism was becoming more widely recognized as a developmental condition, and its rates of diagnosis were rising. People who believed Wakefield's results conflated methylmercury and ethylmercury and promoted the unfounded idea that ethylmercury in vaccines from thimerosal were driving the rising rates of autism.

The Wakefield study was retracted in 2010, and Wakefield was found guilty of dishonesty and flouting ethics protocols by the U.K. General Medical Council, as well as stripped of his medical license. Subsequent studies have not shown a relationship between the MMR vaccine and autism, but despite the absence of evidence, the idea took hold and has proven difficult to dislodge.

Have scientists tested whether thimerosal is safe?

No unbiased research to date has identified toxicity caused by ethylmercury in vaccines or a link between the substance and autism or other developmental concerns — and not from lack of looking.

A 1999 review conducted by the Food and Drug Administration in response to federal guidelines on limiting mercury exposure found no evidence of harm from thimerosal as a vaccine preservative other than rare allergic reactions. Even so, as a precautionary measure in response to concerns about exposure to mercury in infants, the American Academy of Pediatrics and the U.S. Public Health Service issued a joint statement in 1999 recommending removal of thimerosal from vaccines.

At that time, just one childhood vaccine was available only in a version that contained thimerosal as an ingredient. This was a vaccine called DTP, for diphtheria, tetanus and pertussis. Other childhood vaccines were either available only in formulations without thimerosal or could be obtained in versions that did not contain it.

By 2001, U.S. manufacturers had removed thimerosal from almost all vaccines — and from all vaccines in the childhood vaccination schedule.

In 2004, the U.S. Institute of Medicine Immunization Safety Review Committee reviewed over 200 scientific studies and concluded there is no causal relationship between thimerosal-containing vaccines and autism. Additional well-conducted studies reviewed independently by the CDC and by the FDA did not find a link between thimerosal-containing vaccines and autism or neuropsychological delays.

How is thimerosal used today?

In the U.S., most vaccines are now available in single-dose vials or syringes. Thimerosal is found only in multidose vials that are used to supply vaccines for large-scale immunization efforts — specifically, in a small number of influenza vaccines. It is not added to modern childhood vaccines, and people who get a flu vaccine can avoid it by requesting a vaccine supplied in a single-dose vial or syringe.

Thimerosal is still used in vaccines in some other countries to ensure continued availability of necessary vaccines. The World Health Organization continues to affirm that there is no evidence of toxicity in infants, children or adults exposed to thimerosal-containing vaccines.

This edited article is republished from The Conversation under a Creative Commons license. Read the original article.