Which inactivated vaccines contained thimerosal as preservative in 2025 and in what formulations?

1. What thimerosal is and why it was used

Thimerosal is an organomercury compound used for decades as a preservative in multidose vials to prevent bacterial and fungal growth; it metabolizes to ethylmercury and thiosalicylate and has been employed to enable safe use of multidose vaccine presentations especially for seasonal and pandemic influenza campaigns ^{[6] [7]}. Regulators note the compound’s antimicrobial role and that its use declined as single‑dose formulations became available ^[6].

2. Which inactivated vaccines explicitly never contained thimerosal

Federal sources and reviews state directly that several important vaccines “have never contained thimerosal,” including measles‑mumps‑rubella (MMR), varicella (chickenpox), inactivated polio vaccine (IPV), and pneumococcal conjugate vaccines; that language appears in CDC and ACIP materials and in historical reviews ^{[1] [4] [7]}. In short, live viral vaccines and certain inactivated vaccines historically did not rely on thimerosal ^[7].

3. Influenza: the principal inactivated vaccine where thimerosal remained in 2024–25

Reporting and CDC/ACIP documents identify seasonal influenza vaccines as the primary inactivated vaccine still sometimes supplied in multidose vials containing thimerosal in 2024–25; multi‑dose influenza vials facilitated large immunization campaigns and were the typical context for thimerosal use ^{[6] [4]}. Independent fact‑checking summarized CDC data indicating only a small fraction of U.S. flu vaccines in 2024–25 contained thimerosal (FactCheck cited a CDC document saying roughly 4% of flu vaccines did) ^[5].

4. The 2025 policy shift: single‑dose flu shots without thimerosal

In mid‑2025 ACIP recommended that children ≤18 years, pregnant women, and all adults receive seasonal influenza vaccine only in single‑dose formulations free of thimerosal as a preservative; HHS subsequently adopted ACIP’s recommendation to remove thimerosal from all U.S. influenza vaccines ^{[2] [3]}. News outlets and medical societies reported HHS’s July 2025 action and noted the change applied to influenza products distributed in the U.S. ^{[3] [8]}.

5. How common thimerosal formulations were in 2024–25 and practical effect

Multiple items emphasize that thimerosal had already been largely phased out: CDC slides noted most influenza vaccines in 2024–25 were without thimerosal and analyses cited that only a small minority of flu doses used the preservative ^{[4] [5]}. News coverage and medical groups warned the shift was mostly formalizing existing supply patterns ^{[9] [10]}, although some outlets raised concerns about programmatic impacts and public confidence ^[11].

6. Safety debates and competing perspectives

The scientific and regulatory consensus documented in these sources is that studies have not shown thimerosal in vaccines causes neurodevelopmental disorders, and that its removal from many childhood vaccines was a precautionary measure rather than one driven by evidence of harm ^{[6] [7] [12]}. Conversely, political actors and some advisers framed thimerosal as “toxic” and pressed for removal from flu shots; critics of that push warned it reflected anti‑vaccine advocacy influences and risked sowing public confusion despite the lack of new safety evidence implicating thimerosal ^{[9] [10] [11]}.

7. What the available sources do not say

Available sources do not list every specific influenza product name and manufacturer lot‑by‑lot formulation that contained thimerosal in 2024–25; instead they give programmatic percentages and policy positions ^{[4] [5]}. They also do not provide exhaustive global product lists for 2025—most coverage and federal documents focus on U.S. use and recommendations ^{[6] [3]}.

8. Bottom line for readers

If your question is “which inactivated vaccines contained thimerosal in 2025 and in what formulations?”: U.S. sources show that by 2024–25 the primary remaining use was in some multidose inactivated influenza vials (a small minority of doses), while several commonly used vaccines (MMR, varicella, IPV, pneumococcal conjugate) never contained thimerosal; in June–July 2025 ACIP and HHS moved to prefer/require single‑dose, thimerosal‑free seasonal influenza vaccines for the U.S. population ^{[6] [5] [2] [3]}.