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. During the 1990s, the Environmental Protection Agency and the Food and Drug Administration set limits for maximum recommended exposure to methylmercury, particularly for children, pregnant women, and women of childbearing age.
Thimerosal became controversial for two main reasons. Firstly, a discredited report published in The Lancet in 1998 claimed a link between the MMR vaccine and autism, but the study was later found to be falsified. Secondly, concerns about exposure to methylmercury coincided with the rise in autism diagnoses, leading to unfounded fears about thimerosal in vaccines causing autism.
Despite extensive research, no evidence has shown that thimerosal in vaccines causes harm or is linked to autism. The FDA found no harm from thimerosal, but as a precaution, thimerosal was removed from most childhood vaccines in response to public concerns about mercury exposure.
