What Is Thimerosal, the Vaccine Ingredient Currently Under Attack?

A preservative used in a small number of flu shots has come under fire from RFK Jr. and the committee that helps decide which vaccines Americans should get.

An important advisory committee to the federal government has voted this week to withdraw its support for flu shots containing the preservative thimerosal.

The Advisory Committee on Immunization Practices (ACIP) guides vaccine recommendations from the Centers for Disease Control and Prevention (CDC). Earlier this month, health secretary Robert F. Kennedy Jr. fired all 17 members of the committee and appointed eight new members, some of whom have a history of making claims about vaccines that aren’t backed up by scientific research, as reported by NPR.

Kennedy himself has been critical of thimerosal specifically; he wrote a book published in 2014 that called for banning thimerosal in vaccines because of an alleged autism link.

Multiple research studies, however, have found that any concern that thimerosal causes autism is unfounded.

Thimerosal is present in only a small number of flu vaccines. During the 2024–2025 flu season, 96 percent of flu shots administered in the United States were free of thimerosal, according to reporting by the AP.

What Is Thimerosal?

Thimerosal is a mercury-based preservative used in medications or vaccines that come in vials with more than one dose.

Pharmaceutical companies have used thimerosal in vaccines since the 1930s.

“We use it in vials intended for use on multiple patients to prevent growth in case some bacteria are accidentally introduced,” explains Brian Labus, PhD, MPH, an infectious disease epidemiologist and an assistant professor at the University of Nevada, Las Vegas School of Public Health. This might happen, for instance, if there is any bacteria on syringe needles inserted into a vial.

Is Thimerosal Safe?

Numerous studies support the safety of thimerosal-containing vaccines, according to the U.S. Food and Drug Administration (FDA). Only minor reactions, like redness or swelling at the injection site, have been reported.

“It’s been around for a long time, and it’s safe,” says Thomas Russo, MD, a professor and the chief of infectious disease at the University at Buffalo Jacobs School of Medicine and Biomedical Sciences.

Still, people may see that thimerosal is mercury-based and get “a little freaked out,” he acknowledges.

At high levels, methylmercury (the type found in some fish) can be toxic. But thimerosal contains ethylmercury, which the body breaks down and clears much more quickly than methylmercury.

Plus, “The amount contained in these vaccines is minuscule compared with the exposure we get from these compounds in everyday life and things that we ingest,” Dr. Russo says.

For example, “You are exposed to more mercury eating a tuna sandwich than you are getting a vaccine,” Dr. Labus says.

Multiple studies have failed to identify any potential links between thimerosal and autism, including research by the CDC published in 2010.

Very Few Vaccines Contain Thimerosal

Even though there’s no evidence that the small amount of thimerosal in vaccines is unsafe or causes autism, in 1999, the Public Health Service (including the FDA, CDC, and the National Institutes of Health), the American Academy of Pediatrics, the American Academy of Family Physicians, and vaccine manufacturers agreed to reduce or eliminate the use of thimerosal in vaccines for children.

Use of the preservative has also declined in vaccines overall, as manufacturers have reformulated vaccines and developed more single-use doses.

“The only flu shots that contain thimerosal are those that are in vials intended for multiple adult patients,” Labus says. “Individually packaged flu doses do not contain it, nor do any pediatric vaccines.”

These days, most people receive single-dose flu vaccines, Russo says.

Attacks on Thimerosal Could Undermine Confidence in All Vaccines

Since thimerosal is found in only a small number of flu vaccines, Russo says ACIP’s decision will likely not have much of an impact on the vaccine landscape. It also doesn’t mean the preservative isn’t safe, he says.

But he worries that the move could “undermine, in the American public, the view about the safety of vaccines.”

Labus agrees. “Anti-vaxxers on the committee are rolling out their misinformation campaigns’ greatest hits to try to scare people away from getting vaccinated,” he says. He says the committee’s decision has “absolutely no basis in rational, scientific thought.”

Vaccines in the United States are “extremely well vetted” and safe, Russo emphasizes, and everyone should still get a flu shot.

“The benefits of the vaccines approved in this country far outweigh the potential adverse effects,” he says.

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