Thimerosal Reenters Public Debate as ACIP Scrutinizes Use in Influenza Vaccines

Thimerosal (sodium ethylmercurithiosalicylate), a mercury-based preservative that helps suppress bacteria and fungi, was introduced into vaccine manufacturing in the 1930s and used for decades in the US for multi-dose vials of vaccines and other medicines. Vaccine contamination is a serious possible complication of improper storage and production of vaccines, and the addition of thimerosal to vaccines that are used more than once prevents this growth.^1,2

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Thimerosal is Proven Safe in Vaccines

Concerns regarding the possible health effects of thimerosal began to heighten in the late 1990s as the FDA sought to investigate and reduce the amount of mercury in foods and medications, including vaccines. The concerns centered around the presence of methylmercury compounds in thimerosal, which can be toxic at high doses and were alleged to contribute to the development of neurophysiological delays or neurodevelopmental disorders, including autism spectrum disorder (ASD). Out of an abundance of caution, the FDA requested that vaccine manufacturers remove thimerosal from vaccines, and since 2001, all childhood vaccines in the US have been thimerosal-free, with the limited exception of some multi-dose influenza vaccine formulations.^1-3

Critically, extensive research conducted by the CDC and a litany of other investigators have affirmed the safety of thimerosal in multi-dose vaccines and have found no connection with autism or association with neurophysiological delays. A 2010 study conducted by the CDC found that prenatal and early-life exposure to ethyl mercury from thimerosal-containing vaccines was not related to an increased risk of ASD. Furthermore, a recent review of evidence published by CDC affirmed this conclusion, noting that many non-CDC studies that have identified such an association contain major methodological limitations that threaten their reliability.^3,4

In the recent evidence review published by the CDC, they reiterated that thimerosal is already almost entirely removed from the US vaccine supply. In the 2024 to 2025 influenza season, approximately 96% of all influenza vaccines were thimerosal-free. Additionally, the proportion of pregnant women receiving a thimerosal-containing flu vaccine has decreased over time, with only 0.3% of doses in 2024 containing thimerosal. Patients are empowered to check the ingredients of a vaccine they are receiving for the presence of thimerosal, but can be assured that based on the abundance of meaningful evidence available, it is safe.³

Newly Constituted ACIP Scrutinizes Thimerosal in Influenza Vaccines

Still, under Secretary Robert F. Kennedy Jr., the medical establishment of the Department of Health and Human Services (HHS) has begun to relitigate the issue of thimerosal. At the recent meeting of the Advisory Committee on Immunization Practices (ACIP), the first since Kennedy purged the committee of its 17 sitting members and replaced them with 8 new experts, the committee discussed the topic of thimerosal’s inclusion in influenza vaccines and set up votes on a recommendation for thimerosal-free influenza vaccines for adults, children, and pregnant women.⁵

Part of the discussion involved a presentation from Lyn Redwood, RN, MSN, a nurse practitioner and president emerita of Children’s Health Defense who was reportedly hired as a special employee at HHS. The organization, which was founded by Kennedy and states on their website their mission of “ending childhood health epidemics by eliminating toxic exposure,” has widely been described as an antivaccine advocacy group. Redwood has previously authored literature falsely attributing autism to mercury toxicity in medicines, including vaccines.^6,7

Her presentation at the ACIP meeting had a similar focus. The presentation centered on thimerosal’s alleged ineffectiveness as a preservative and cited evidence claiming that exposure to vaccine-level thimerosal can cross the blood-brain barrier and result in significant levels of mercury in the brain, inducing high toxicities in children. Redwood said that the US has enough thimerosal-free influenza vaccines to recommend that all individuals receive such formulations.⁸

Notably, an earlier version of Redwood’s presentation referred to a study, titled “Low-level neonatal thimerosal exposure: Long-term consequences in the brain,” that did not exist, according to Robert Berman, professor emeritus at UC Davis, who was claimed to have coauthored the study and published it in Neurotoxicology in 2008. Instead, Berman coauthored a similarly named study in a different journal and came to differing conclusions than Redwood. In an exclusive statement to Reuters, Berman decried the misrepresentation and claimed he did not endorse the use of his research.^8,9

The presentation was updated with a new slide that did not contain the nonexistent study. The New York Times reported that, in addition to this error, Redwood’s presentation incorrectly claimed that the influenza vaccine contains double the amount of thimerosal that it does. This slide was also removed, according to The Times, but the information was reiterated further along in the presentation. The insights presented by Redwood were challenged by liaisons to ACIP from several organizations, and from H. Cody Meissner, MD, ACIP member and professor of pediatrics at Dartmouth Geisel School of Medicine.^7,8,10

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"The risk from influenza is so much greater than the nonexistent, as far as we know, risk from thimerosal," Meissner said. "I would hate for a person not to receive the flu vaccine because the only available vaccine contains thimerosal."¹⁰

With New Recommendation, Pharmacists Must Educate Patients

Still, ACIP voted in favor of a recommendation that adults, children, and pregnant women should not receive thimerosal-containing seasonal influenza vaccines, instead recommending that they receive single-dose, thimerosal-free vaccines. The recommendations—which are currently awaiting confirmation in the US Senate—are not final until Kennedy or the CDC director sign off on them, and do not mean that thimerosal-containing vaccines will no longer be available to patients; however, the recommendation raises serious concerns about access to and insurance coverage for influenza vaccines in the upcoming 2025 to 2026 respiratory season.¹⁰

“Our votes are recommendations, but we know that some may perceive them as mandates, so we take this responsibility very seriously,” the members of ACIP said in a joint statement following the vote. “We pledge not to hold a vote if there is not sufficient information to enable evaluation of the risks and benefits.”¹¹

With these new recommendations poised to be certified, pharmacists are in a critical position of explaining to patients how the decision may impact their access to an influenza vaccine and reaffirm that thimerosal is safe in influenza vaccines. In a guest blog on the website of the American Pharmacists Association, Col (Ret) John D. Gräbenstein, RPh, PhD, FAPhA, explained that the evidence is clear that thimerosal is not associated with any hazards, and that counseling patients on these facts is one of the best courses a pharmacist can take.²

“Remember all the lessons of good communication about vaccine benefits and risks from the COVID-19 vaccine rollout,” Gräbenstein said. “Listen with respect. Understand the basis for your patients’ concerns. Recall that connections are more important than statistics. Convey concern for their health.”²

Ultimately, if a patient is adamant to not accept a vaccine with thimerosal, Gräbenstein said that a pharmacist can simply offer them a single-dose influenza vaccine that does not contain thimerosal. Above all else, ensuring patients receive routine, up-to-date protection against influenza is essential, with the risks of influenza far outweighing any purported risks of thimerosal. Pharmacists should continue to advocate for vaccination and encourage patients to protect themselves and others with a vaccine.²

REFERENCES

1. CDC—Vaccine safety. Thimerosal and vaccines. Last Updated December 19, 2024. Accessed June 27, 2025. https://www.cdc.gov/vaccine-safety/about/thimerosal.html

2. Gräbenstein JD. Thimerosal: Quality evidence of no harm. APhA. Published June 25, 2025. Accessed June 27, 2025. https://www.pharmacist.com/Blogs/Guest-Spotlights/Article/thimerosal-quality-evidence-of-no-harm

3. CDC. Thimerosal-containing vaccines and neurodevelopmental outcomes—Review of the evidence. Published June 23, 2025. Archive Accessible Online via Wayback Machine: https://web.archive.org/web/20250624164828/https://www.cdc.gov/acip/downloads/slides-2025-06-25-26/Thimerosal-Containing-Vaccines-Summary-of-Evidence-508.pdf. Accessed June 27, 2025.

4. Price CS, Thompson WW, Goodson B, et al. Prenatal and infant exposure to thimerosal from vaccines and immunoglobulins. Pediatrics. 2010;126(4):656-664. doi:10.1542/peds.2010-0309

5. Halpern L. Department of Health and Human Services reconstitutes ACIP, removing 17 sitting members. Pharmacy Times. Published June 10, 2025. Accessed June 27, 2025. https://www.pharmacytimes.com/view/department-of-health-and-human-services-reconstitutes-acip-removing-17-sitting-members

6. Bendix A, Edwards E. Anti-vaccine activist presents data to Kennedy’s reshaped CDC advisory panel. NBC News. Published June 26, 2025. Accessed June 27, 2025. https://www.nbcnews.com/health/health-news/anti-vaccine-activist-presents-data-rfk-jrs-reshaped-cdc-advisory-pane-rcna215201

7. Mandavilli A. Kennedy’s new advisers rescind recommendations for some flu vaccines. The New York Times. Published June 26, 2025. Accessed June 27, 2025. https://www.nytimes.com/2025/06/26/health/rfk-jr-vaccines-acip-cdc.html

8. Redwood L. Thimerosal as a vaccine preservative—Prepared for the Advisory Committee for Immunization Practices. Presented: June 26, 2025. Atlanta, Georgia. Accessed June 27, 2025. Accessible Online at https://www.cdc.gov/acip/downloads/slides-2025-06-25-26/05-influenza-redwood-508.pdf

9. Erman M. Exclusive: US CDC vaccine presentation cites study that does not exist, author says. Reuters. Published June 24, 2025. Accessed June 27, 2025. https://www.reuters.com/business/finance/us-cdc-report-shows-no-evidence-linking-thimerosal-containing-vaccines-autism-2025-06-24/

10. Halpern L. ACIP recommends adults receive seasonal, single-dose influenza vaccines free of thimerosal. Pharmacy Times. Published June 26, 2025. Accessed June 27, 2025. https://www.pharmacytimes.com/view/acip-recommends-adults-receive-seasonal-single-dose-influenza-vaccines-free-of-thimerosal

11. CDC. CDC’s Advisory Committee on Immunization concludes meeting with joint statement. News Release. Released June 26, 2025. Accessed June 27, 2025. https://www.cdc.gov/media/releases/2025/2025-cdcs-advisory-committee-on-immunization-concludes-meeting-with-joint-statement.html