ACIP Votes 5–1 to Recommend Against Thimerosal in Flu Vaccines

The Advisory Committee on Immunization Practices (ACIP) recommended against influenza vaccines with Thimerosal as an adjuvant during the two-day meeting on Wednesday and Thursday. The CDC web page on Thimerosal in vaccines hasn’t been updated since December 2024 and states, “Thimerosal contains ethylmercury, which is cleared from the human body more quickly than methylmercury, and is therefore less likely to cause any harm.”

The ACIP meeting came just days after HHS Secretary Robert F. Kennedy Jr. released the 17 sitting members and replaced them with eight new members, including Dr. Robert Malone and Dr. Martin Kulldorff. Michael Ross, who was initially appointed to the ACIP, withdrew his name during the conflict of interest review. The seven remaining members were all present for the two-day meeting.

There was a presentation to the committee before a vote took place regarding Thimerosal. “Mercury is the third most toxic element on Earth only behind polonium and plutonium, and it has no physiological role in the human body,” said Lyn Redwood, RN, MSN, FP, during a presentation at the ACIP meeting. “Thimerosal was grandfathered for use without adequate safety testing by the FDA and is not generally recognized as being safe or effective by the FDA over-the-counter division since 1998.”

Thimerosal has not been used in childhood vaccinations since 2001, but it is still used as an ingredient in three of the 12 licensed influenza vaccinations in the United States. In 1999, the US Public Health Service (PHS) and the American Academy of Pediatrics (AAP) issued a joint statement calling for all thimerosal-containing vaccines to be removed from the market as soon as possible.

The agencies said the risk of not vaccinating was a greater risk, but the risk from Thimerosal can be removed if vaccine manufacturers create alternative options. The statement said, “Similar conclusions were reached this year in a meeting attended by European regulatory agencies, European vaccine manufacturers, and FDA, which examined the use of thimerosal-containing vaccines produced or sold in European countries.”

Afluria and Flucelvax are two of the influenza vaccines that still contain Thimerosal and are manufactured by Seqirus. Fluzone also contains Thimerosal and is manufactured by Sanofi. These three vaccines reportedly make up about 4% of the influenza vaccine supply.

Redwood’s presentation included information from a 2023 study about the cytotoxicity of Thimerosal. The authors concluded, “Overall, our study revealed Thimerosal was 333-fold more cytotoxic to human and animal cells as compared to bacterial and fungal cells. Our results promote more study on Thimerosal toxicity and its antimicrobial effectiveness to obtain more safe concentrations in biopharmaceuticals.”

Three separate votes took place, and five members voted to recommend against children, pregnant women, and adults receiving influenza vaccines containing Thimerosal. One member, Vicky Pebsworth, abstained from all three votes. She said two things were asked in the same question. The question recommends seasonal influenza vaccines for all adults, but also calls for those vaccines not to contain Thimerosal as a preservative. She also said she doesn’t believe vaccines already approved by the FDA should be restricted by the committee.

Only one member voted against the three motions: Dr. Cody Meissner. “There is no scientific evidence that thimerosal has caused a problem,” Meissner said. “No study has ever indicated any harm from Thimerosal. The risk from influenza is so much greater than the non-existent, as far as we know, risk from Thimerosal.”

Meissner and others state that science has not proven any harm from Thimerosal and that studies presented by Redwood have been debunked. Redwood was also criticized for conflating ethylmercury, the kind found in thimerosal vaccines, with methylmercury. One of the studies cited by Redwood was from 2005, which tested blood and brain mercury levels in infant monkeys.

One group was exposed to ethylmercury, and the other to methyl mercury. The study found 71% of the “inorganic Hg” or ethylmercury reached the brains of the monkeys in the study, compared to 10% for the methylmercury (MeHg) group. The researchers also tracked kidney exposure and found 71% of mercury in the ethylmercury group while the MeHg group had 40%.

Meissner and others criticized Redwood’s presentation for emphasizing the higher proportion of inorganic mercury in the brain from Thimerosal without acknowledging ethylmercury’s faster clearance from infants. The Burbacher study reported that ethylmercury from Thimerosal has a blood half-life of 6.9 days in infant monkeys, similar to the ~7-day half-life in human infants reported by Pichichero et al. in 2002. In contrast, methylmercury (MeHg) has a blood half-life of 19.1 days, resulting in greater accumulation in the brain and more significant health concerns due to its slower clearance and known neurotoxicity at high doses, despite having a lower proportion of inorganic mercury in the brain.

Kulldorf said it is a good idea to remove mercury and Thimerosal from vaccines because it is a well-known toxin. “We know that mercury is a toxin,” Dr. Kulldorf said. “We cannot completely avoid exposure to mercury, but it’s a cumulative issue. We already have some exposure to mercury from other sources. Even if the amount available in the vaccine – maybe that amount is safe – but that’s not the amount we’re exposed to. If we care about public health, we should try to minimize exposure to mercury. I know that FDA do not allow mercury, for example, in skin products. If that’s the only exposure you had, that would probably be safe, but we want to minimize the cumulative exposure.”

The committee also voted 5-2 to approve one dose of clesrovimab, a monoclonal antibody to protect infants under 8 months of age from RSV. In a 7-0 vote, the committee recommended that clesrovimab be available as part of the Vaccines for Children Program, which provides vaccinations to families who may not be able to afford them.