A preservative removed from childhood vaccines 20 years ago is still causing controversy today − a drug safety expert explains

• Thimerosal, a mercury-based chemical used as a preservative in some vaccines, has been removed from childhood vaccines for over 20 years due to concerns about its safety.
• A discredited study published in 1989 falsely linked thimerosal to autism, but subsequent studies have found no evidence of a causal relationship between the two.
• The Advisory Committee on Immunization Practices (ACIP) has voted to cease recommending the use of thimerosal in flu vaccines, citing a lack of evidence supporting its safety.
• Despite the removal of thimerosal from childhood vaccines, some parents still worry about its potential health risks, and misinformation about its safety has persisted.
• Public health experts warn that the ACIP’s decision may shake public trust and sow confusion about vaccine safety, highlighting the need for continued education and evidence-based information about vaccine ingredients and their effects.

An expert committee that advises the Centers for Disease Control and Prevention voted on June 26, 2025, to cease recommending the use of a mercury-based chemical called thimerosal in flu vaccines. Only a small number of flu vaccines – ones that are produced in multi-dose vials – currently contain thimerosal.

Thimerosal is almost never used in vaccines anymore, but vaccine skeptics have falsely claimed it carries health risks to the brain. Public health experts have raised concerns that the committee’s action against thimerosal may shake public trust and sow confusion about the safety of vaccines.

The committee, called the Advisory Committee on Immunization Practices, or ACIP, was meeting for the first time since Health Secretary Robert F. Kennedy Jr. abruptly replaced its 17 members with eight handpicked ones on June 11.

The committee generally discusses and votes on recommendations for specific vaccines. For this meeting, vaccines for COVID-19, human papillomavirus, influenza and other infectious diseases were on the schedule.

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.

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 proved 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 multi-dose 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 article was updated to include ACIP’s vaccine recommendations.

Terri Levien does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

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