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Thimerasol still being used in inactivated vaccines
Executive summary
Thimerosal (also spelled thiomersal) is no longer present in most routinely recommended childhood vaccines in the United States and, before 2025 actions, was found mainly in some multi-dose influenza (flu) vaccine vials; during the 2024–25 season roughly 94–96% of U.S. flu doses were thimerosal-free [1] [2]. In mid‑2025 the Advisory Committee on Immunization Practices (ACIP) recommended avoiding thimerosal-containing flu vaccines and HHS moved to adopt that guidance to remove thimerosal from U.S. influenza vaccines [3] [4].
1. What thimerosal is and why it was used
Thimerosal is a mercury‑containing organomercurial compound used as an antimicrobial preservative in multi‑dose vials to prevent bacterial or fungal contamination when a needle punctures a shared vial; regulators say that property has contributed to safe use of multi‑dose vaccines, particularly for influenza and in settings where single‑dose vials are less practical [5] [6].
2. The historical shift away from routine childhood use
Starting around 1999, U.S. public‑health agencies and manufacturers moved to remove or reduce thimerosal from routine childhood vaccine formulations as a precaution; by the early 2000s thimerosal had been removed or reduced to trace amounts in all routinely recommended childhood vaccines, with at least one thimerosal‑free formulation available for each dose on the schedule [7] [8].
3. Where thimerosal remained as of 2024–25
Even after removal from childhood schedules, thimerosal persisted primarily in some multi‑dose flu vaccine vials. Public‑facing summaries from CDC/ACIP noted that most flu vaccines were already thimerosal‑free—around 94–96% in recent seasons—while a small fraction of multi‑dose influenza formulations contained the preservative [1] [2].
4. The 2025 ACIP recommendation and HHS action
In June 2025 ACIP advisers recommended against using flu shots that contain thimerosal, and HHS subsequently adopted ACIP’s recommendation to remove thimerosal from all influenza vaccines distributed in the United States [3] [4]. Reporting places this action in the context of a reconstituted ACIP and heightened debate about the preservative’s risks and public trust issues [2] [3].
5. Scientific consensus on safety vs. political and public concerns
Major medical organizations and prior safety reviews have found no credible evidence linking thimerosal in vaccines to autism or neurodevelopmental disorders and describe the research base as extensive; these reviews also note that thimerosal‑related reactions are generally limited to minor local effects [9] [8] [7]. At the same time, some policymakers and new advisory members in 2025 have framed thimerosal as a toxin and pushed for removal—actions that commentators warn could fuel confusion despite the long record of safety studies [9] [10] [2].
6. Practical tradeoffs and public‑health implications
Regulators note the documented antimicrobial role of thimerosal facilitates multi‑dose vial use, which can be important for rapid distribution during seasonal or pandemic responses and in some global settings [5]. Public‑health reporting and experts flagged two competing considerations: reducing a preservative that public opinion may find objectionable, and preserving multi‑dose vaccine logistics that depend on safe preservatives [5] [10].
7. What the sources do and do not say about current use
CDC and FDA background material state that thimerosal “has been used” in multi‑dose vials for decades and that several U.S. vaccines (MMR, varicella, inactivated polio, pneumococcal conjugate) have never contained it; ACIP slides and public reporting document that, prior to the 2025 ACIP action, a small number of influenza vaccines used multi‑dose vials with thimerosal while most doses were thimerosal‑free [6] [1] [7]. Available sources do not mention that thimerosal is currently used in routine childhood vaccines in the U.S.; rather, they indicate it was effectively absent from the childhood schedule and concentrated in some flu formulations before the 2025 policy change [7] [8].
8. How to interpret the dispute: science, perception, and agenda
Scientific reviews emphasize extensive study without demonstrated neurodevelopmental harm from thimerosal at vaccine exposure levels [9] [7]. Media coverage and public comments around the 2025 ACIP decision highlight competing motives: some members and public officials advocating removal frame it as precautionary or responsive to public concern, while vaccine scientists warn that politicized moves could undermine trust and are driven in part by advisory‑panel turnover and vocal anti‑vaccine actors [2] [10] [3].
9. Practical takeaways for patients and clinicians
If you or a patient wish to avoid thimerosal, most flu vaccines in recent seasons were already thimerosal‑free and regulators said shifting away from multi‑dose vials should not limit vaccine availability for the upcoming season [2] [5]. For specific clinical guidance, the sources recommend discussing options with a healthcare provider; the NFID and CDC materials emphasize that the known risks of influenza typically outweigh the very low risk profile associated with preserved multi‑dose vaccines [8] [6].
Limitations: this summary relies solely on the provided documents, which focus on U.S. policy through mid‑2025; sources do not provide global current‑use inventories or later 2025–2026 implementation details beyond HHS adoption of ACIP recommendations [4] [1].