The seven new members of the Advisory Committee on Immunization Practices revived a decades-old debate with a Thursday vote in favor of recommending seasonal flu vaccines — only if they're free of the preservative thimerosal.

It's in relatively few vaccines in the U.S. today, though organizations including the American Academy of Pediatrics, a leading institution that recommends health policies for children in the U.S., have stated a lack of evidence that thimerosal causes harm.

A federal board historically made up of more than a dozen experts in infectious disease, pediatrics, public health and vaccination, the Advisory Committee on Immunization Practices (ACIP) periodically reviews immunization guidelines and safety profiles and publishes recommendations that further inform the Centers for Disease Control and Prevention guidelines released to the public.

Health Secretary Robert F. Kennedy Jr. made what he called "a clean sweep" of the committee, in an early June press release.

"ACIP's new members will prioritize public health and evidence-based medicine," he said in the release.

By a vote of five to one, with one member abstaining, the recommendations, according to a June 26 statement from the Department of Health and Human Services, specifically names seasonal flu vaccines for three groups: those under 18, pregnant women and all adults. The move has disturbed medical experts, many who see it as a political win for the vaccine opposition movement.

Here's what to know about thimerosal:

What is thimerosal?

Thimerosal is a preservative used in some multidose vaccines to prevent contamination from bacteria.

"Any biologic product you're going to give to persons, you need to keep it safe with preservatives as you're putting multiple needle jabs into it," said Richard Zimmerman, professor of family medicine and public health at the University of Pittsburgh and a Pittsburgh-based physician. "The vial could be contaminated."

The American Academy of Pediatrics released a statement in 1999 asking vaccine manufacturers to remove it from vaccines out of an abundance of caution, because thimerosal is a mercury-based product, and the public was concerned about infant exposure to mercury.

"AAP said, 'Mercury is a heavy metal; it would be best if we could avoid heavy metals in infants who have a small body weight,'" said Dr. Zimmerman, who was a liaison to ACIP in 1999. "There was a lack of data on thimerosal at the time. The decision was to move away out of an abundance of caution."

Because of this, the vast majority of vaccines in the U.S. today do not contain thimerosal.

Physicians have argued that the claims of thimerosal's harms related to mercury have been taken out of context when linking it to autism and neurodevelopmental conditions.

In its statement, AAP said there was no evidence of thimerosal causing harm. The institution further codified this in a May 2025 fact check article on vaccines.

"There are no data or evidence of any harm caused by the level of exposure that some children may have encountered in following the existing immunization schedule," read the 1999 statement. "Infants and children who have received thimerosal-containing vaccines do not need to be tested for mercury exposure.

"The large risks of not vaccinating children far outweigh the unknown and probably much smaller risk, if any, of cumulative exposure to thimerosal-containing vaccines over the first 6 months of life," it continued.

What are the claims that thimerosal is dangerous?

The anti-vaccine movement has railed against thimerosal for decades, when the movement was still considered fringe.

In Thursday's ACIP meeting, the only scientific presentation regarding thimerosal came from Lyn Redwood, a registered nurse and former president of Children's Health Defense, a nonprofit that aims to restrict access to vaccines, claiming "toxic exposure."

The CHD site hosts multiple pages attempting to link the measles, mumps and rubella (MMR) vaccine, for instance, with autism. One page calls it a "poison pill" and states long-debunked claims about its links to autism as recent as April. Kennedy is the founder of Children's Health Defense.

Martin Kulldorff, the new chair of ACIP, in the meeting mentioned the mercury-containing aspect of thimerosal.

"Even if the amount available in the vaccine, maybe it's safe, but (exposure) is cumulative," he said. "There is a need, if we care about public health, we should try to minimize exposure to mercury."

Kulldorff, a Swedish biostatistician, was dismissed from his position at Harvard Medical School for refusing the COVID-19 vaccine, saying he has a weakened immune system. He was also a paid expert witness in testimony against pharmaceutical company Merck's Gardasil vaccine, Reuters reported. He stated in the ACIP meeting he had no conflicts of interest.

"Removing a known neurotoxin from being injected into our most vulnerable populations is a good place to start with Making America Healthy Again," read a bullet point on Redwood's slides.

Except thimerosal is not a neurotoxin.

Thimerosal is broken down into ethylmercury in the body, as opposed to methylmercury, a known toxin found in small amounts in fish. Ethylmercury leaves the body much more quickly; it does not accumulate as methylmercury does, and "is not associated with the high neurotoxicity that methylmercury is," said Cody Meissner, a professor of pediatrics at Dartmouth Geisel School of Medicine, senior vaccine and biological development analyst for HHS and a new ACIP member, during Thursday's meeting.

Meissner questioned the spotlight on thimerosal and pushed back against other ACIP members' assertions of its harm.

"I'm not quite sure how to respond to this presentation," he said. "This is an old issue that has been addressed in the past.

"As many vaccine experts have said, all vaccines routinely recommended for young children in the United States are available in formulations that do not contain thimerosal as a preservative," he added. "Of all the issues that I think ACIP needs to focus on, this is not a big issue."

The anti-vaccine movement has long tried to link thimerosal to autism, though no valid correlation has been established.

Kennedy edited a book, "Thimerosal: Let The Science Speak" (Simon & Schuster), in 2015, stating that thimerosal causes autism.

"Thimerosal was never found to be a problem," said Zimmerman. "As thimerosal was removed, the autism rate increased in the U.S. There's no relationship. I just don't think this is a big deal."

Will the thimerosal ruling impact access to vaccines?

Physicians interviewed didn't think the thimerosal ruling would substantially impact access to flu vaccines in the U.S. because a vast majority of them already don't contain the preservative.

Multidose vials are more common in vaccine administration in other countries due to the cost-saving benefits and easier storage space in refrigerators. As ACIP's recommendations reverberate, though, the ruling may impact vaccine uptake across the globe, said Meissner in the meeting.

"The recommendations the ACIP makes are followed among many countries around the world," he said. "And removing thimerosal from all vaccines used in other countries, for example, is going to reduce access to these vaccines; it will increase cost. It's important to note no study has ever indicated any harm from thimerosal. It's been used in vaccines since before World War II."

Less-so a ripple effect in the U.S., the implications from the meeting will further promote distrust in vaccines overall, said Amesh Adalja, senior scholar of the Johns Hopkins Center for Health Security and a Pittsburgh-based physician.

"Anytime thimerosal gets smeared, the global vaccine effort is impacted," he said. "This can only be seen as a purely political move by the anti-vaccine movement. The whole action is just meant to sow distrust in vaccines."

AAP said in a Thursday statement it would begin publishing its own independent vaccine guidelines, as the creation of federal vaccine policy is "no longer a credible process."

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