A Misguided Recommendation To Remove Thimerosal From U.S. Influenza Vaccines

(A version of this article was published in the October 2025 issue of Public Citizen’s Health Letter)

In June 2025 the Advisory Committee on Immunization Practices (ACIP) at the Centers for Disease Control and Prevention recommended that children, pregnant women, and adults receive only single-dose formulations of annual influenza (flu) vaccines that are free of thimerosal,[1] a mercury-based preservative that prevents the growth of bacteria and fungi.

The recommendation followed Health and Human Services Secretary Robert F. Kennedy, Jr.’s unprecedented firing and replacement of all ACIP members.[2] Subsequently, Kennedy accepted the thimerosal recommendation,[3] which the fired committee members would likely never have made. The recommendation is now federal health policy.

The thimerosal recommendation was misguided for several reasons. First, single-dose formulations of annual influenza vaccines are available in the United States, widely used, and free of thimerosal.[4] Second, there is no evidence of harm from low doses of thimerosal in multidose vials, other than minor injection-site reactions.[5] Third, thimerosal has not been used in vaccines for children since the early 2000s;[6] all vaccine formulations for children six years of age and younger are available without the preservative.[7]

Finally, although Kennedy’s action may have little practical effect, it was not based on scientific evidence, thus undermining trust in federal recommendations about vaccines. The same point was made by the American Academy of Pediatrics. After reiterating that “extensive research proves that thimerosal is a safe ingredient in vaccines,” the academy stated that “[b]anning vaccine ingredients without solid scientific reasons sets a dangerous precedent and ultimately makes children less safe.”[8]

The FDA and CDC websites remain among the best sources of information about the safety of thimerosal in vaccines. As their information makes clear, there are two types of mercury: methylmercury and ethylmercury. Methylmercury, which is found in some types of fish, can be toxic to people at high exposure levels. Ethylmercury, the type of mercury in thimerosal, is cleared from the body more quickly than methylmercury and less likely to cause harm, with no evidence of harm from the doses used in vaccines. Specifically, there is no evidence of a connection between thimerosal as used in medical products with either autism or neuropsychological delays.[9],[10] A typical vaccine dose containing thimerosal as a preservative has about the same amount of elemental mercury as a 3-ounce can of tuna fish.[11]

In the United States, vaccine manufacturers may replace multidose vials of influenza vaccines containing thimerosal, ensuring that supplies remain uninterrupted.[12] Globally, removing thimerosal from vaccines could depress their uptake in developing countries because multidose vials are more practical and affordable.[13] However, the World Health Organization has not changed its view that thimerosal is safe in multidose vials,[14] suggesting that the same multidose vials that are used now will continue to be used internationally.

The real concern is what happens next. At present, ACIP has seven members, all hand-picked by Secretary Kennedy and often in alignment with his views about vaccines.[15] Absent a successful legal challenge to the ACIP firings or a restoration of nonpartisan scientific expertise as the essential qualification for membership, ACIP may, at a future meeting, make vaccine recommendations that are even more damaging, and this pattern may continue indefinitely. The ongoing threat to vaccination in the United States and public health is very real.
 

References

[1] CDC Advisory Committee on Immunization Practices (ACIP). ACIP Recommendations. July 28, 2025. https://www.cdc.gov/acip/vaccine-recommendations/index.html. Accessed August 4, 2025.

[2] Cirruzzo C, Cueto I, Joseph A, Branswell H. RFK Jr. names new members of CDC’s vaccine advisory panel. STAT News. June 11, 2025. https://www.statnews.com/2025/06/11/rfk-jr-names-new-acip-members-replaces-cdc-vaccine-experts-he-just-fired/. Accessed August 4, 2025.

[3] U.S. Department of Health and Human Services. HHS adopts ACIP recommendation to remove thimerosal from all U.S. influenza vaccines. July 23, 2025. https://www.hhs.gov/press-room/thimerosal-mercury-removed-from-us-flu-vaccines-acip.html. Accessed August 4, 2025.

[4] Centers for Disease Control and Prevention. Vaccine Safety: Thimerosal and vaccines. December 19, 2024. https://www.cdc.gov/vaccine-safety/about/thimerosal.html. Accessed August 4, 2025.

[5] Ibid.

[6] Ibid.

[7] Food and Drug Administration. Thimerosal and vaccines. January 15, 2025. https://www.fda.gov/vaccines-blood-biologics/safety-availability-biologics/thimerosal-and-vaccines. Accessed August 4, 2025.

[8] American Academy of Pediatrics. Fact-checked: extensive research shows thimerosal is safe. July 15, 2025. https://www.aap.org/en/news-room/fact-checked/fact-checked-extensive-research-shows-thimerosal-is-safe/. Accessed August 4, 2025.

[9] Food and Drug Administration. Thimerosal and vaccines. January 15, 2025. https://www.fda.gov/vaccines-blood-biologics/safety-availability-biologics/thimerosal-and-vaccines. Accessed August 4, 2025.

[10] Centers for Disease Control and Prevention. Vaccine Safety: Thimerosal and vaccines. December 19, 2024. https://www.cdc.gov/vaccine-safety/about/thimerosal.html. Accessed August 4, 2025.

[11] Food and Drug Administration. Thimerosal and vaccines. January 15, 2025. https://www.fda.gov/vaccines-blood-biologics/safety-availability-biologics/thimerosal-and-vaccines. Accessed August 4, 2025.

[12] U.S. Department of Health and Human Services. HHS adopts ACIP recommendation to remove thimerosal from all U.S. influenza vaccines. July 23, 2025. https://www.hhs.gov/press-room/thimerosal-mercury-removed-from-us-flu-vaccines-acip.html. Accessed August 4, 2025.

[13] Branswell, H. HHS secretary RFK Jr. accepts recommendations to drop thimerosal from U.S. flu vaccines. STAT News. July 23, 2025. https://www.statnews.com/2025/07/23/kennedy-approves-acip-recommendation-thimerosal-removed-from-flu-vaccines/#:~:text=HHS%20Secretary%20RFK%20Jr.%20accepts,thimerosal%20from%20U.S.%20flu%20vaccines. Accessed August 4, 2025.

[14] WHO says ‘no evidence of harm’ from thimerosal. Reuters. June 27, 2025. https://www.reuters.com/business/healthcare-pharmaceuticals/who-says-no-evidence-harm-thimerosal-2025-06-27/. Accessed August 4, 2025.

[15] STAT Staff. A closer looks the new members of the CDC vaccine advisory panel. Statnews. June 11, 2025. https://www.statnews.com/2025/06/11/rfk-jr-vaccine-advisory-panel-new-acip-members-include-mrna-skeptics-covid-critics/. Accessed August 4, 2025.