Editor’s Note (6/27/25): On June 26, the Advisory Committee on Immunization Practices voted to reverse a longstanding recommendation regarding flu vaccine formulations that include thimerosal. During the 2024–2025 flu season, these vaccines accounted for only 4 percent of the total supply and were mostly administered to adults.

This week, the Advisory Committee on Immunization Practices at the Centers for Disease Control and Prevention convened to discuss several vaccine recommendations. Recently, Secretary of Health and Human Services Robert F. Kennedy Jr. expanded the meeting’s agenda to include a review of thimerosal, a mercury-based compound used in some vaccines. It’s important to note that all pediatric vaccines have eliminated thimerosal, and extensive research has confirmed that it does not lead to neurodevelopmental disorders. The type of mercury in thimerosal is processed quickly and effectively by the human body, which is a key reason it remains in use in certain adult vaccines.

Understanding Thimerosal and Its Role in Vaccines

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Thimerosal has been used as a preservative in vaccines since the 1930s due to its potent antiseptic properties, which prevent the growth of harmful fungi and bacteria. Comprising about 50 percent ethylmercury by weight, thimerosal has raised concerns due to the general toxicity of mercury, a fact that is well-recognized from warnings about high mercury levels in certain fish like tuna.

“Mercury’s toxicity is well-known,” states Ryan Marino, a medical toxicologist at University Hospitals in Cleveland. “However, it’s crucial to understand that different forms of mercury have vastly different levels of toxicity.”

According to Marino, mercury is a common environmental element released naturally by volcanoes, forest fires, and rock erosion, but it is predominantly introduced into the environment through human activities such as mining, burning coal, and from industrial waste.

Comparing Ethylmercury with Other Forms of Mercury

In nature, inorganic mercury is transformed into methylmercury by microorganisms, a compound that accumulates in aquatic life and magnifies through the food chain, particularly in large fish like sharks and tuna. This bioaccumulation is why certain fish consumption is regulated by governmental health advisories.

However, the mercury in thimerosal is ethylmercury, which differs significantly from methylmercury due to its chemical structure. While both contain mercury, their effects on the body are not the same. Ethylmercury, for example, has a much shorter half-life in the human body, lasting only about a week, which allows for quicker elimination from the body.

“Ethylmercury does not accumulate in the body like methylmercury,” Marino explains. “The quantity present in vaccines is cleared very rapidly by the body. It would take an unrealistic number of vaccines at one time to pose a health risk.”

Timeline and Reasoning Behind the Removal of Thimerosal from Vaccines

In efforts to reduce mercury exposure, the U.S. government enacted the Food and Drug Administration Modernization Act of 1997, which necessitated the FDA to identify and quantify mercury content in products. At that time, thimerosal was found in three pediatric vaccines. Although no harm was demonstrated, the levels of ethylmercury exceeded some federal safety guidelines for methylmercury, prompting a reevaluation.

“The assumption that ethylmercury behaved the same as methylmercury has been proven incorrect,” asserts Marino. In 1999, amidst varying safety guidelines and the theoretical risk of harm, the AAP and the U.S. Public Health Service recommended the removal of thimerosal from pediatric vaccines, a policy fully implemented by 2001 except in certain flu vaccines.

Today, thimerosal is used only in multi-dose flu vaccine vials to prevent contamination, as these vials are used multiple times. Trace amounts of thimerosal might still be found in other vaccines due to the manufacturing process, but these amounts are negligible and considered safe.

New Insights on Thimerosal

Post-removal studies have consistently shown that thimerosal in vaccines does not increase the risk of neurodevelopmental disorders or other health issues. In 2004, the Institute of Medicine, now known as the National Academy of Medicine, reviewed extensive epidemiological studies and found no link between thimerosal-containing vaccines and autism.

Marino and Halsey both agree that removing thimerosal was prudent, though opinions vary on its impact on public perception. Marino suggests that attempting to mitigate unfounded fears might have inadvertently fueled misperceptions, while Halsey believes that not removing it could have severely damaged public trust.