Guest blog from Col (Ret) John D. Gräbenstein, RPh, PhD, FAPhA
Thimerosal is back in the news as vaccine safety becomes a common topic again.
Recently, 17 distinguished members of the CDC Advisory Committee on Immunization Practices (ACIP) were abruptly dismissed on June 9, 2025. The few replacements named so far pale in comparison to the deep scientific expertise and credibility of the previous panel. CDC career professionals who support ACIP have been reassigned. Agenda items with no apparent logical basis are listed for the June 26 meeting, including “Proposed recommendations regarding thimerosal containing influenza vaccine.”
Thimerosal is a mercury-based preservative (sodium ethylmercurithiosalicylate) added to multi-dose vials since the 1930s. The preservative helps suppress bacteria that might be introduced into a vial as needles puncture the rubber stopper and doses are withdrawn.
In 1997, the U.S. Congress triggered FDA to identify and reduce the amount of mercury in food and medications. This effort was driven by concerns about methylmercury compounds found in fish and seafood. At high levels, methylmercury can be toxic, especially regarding neurologic development of infants. On the other hand, thimerosal is an ethylmercury compound that does not cause these neurologic effects. Ethylmercury compounds are more readily metabolized and excreted by the human body.
Even so, starting in 1999, thimerosal was being reduced or eliminated in vaccines as a precautionary measure. Vaccine manufacturers and vaccine purchasers accelerated the shift from multi-dose to single-dose containers. By 2002, pediatric vaccine containers did not contain any thimerosal preservative. The only vaccine exception was multi-dose containers of influenza vaccine, typically purchased for large-scale adult vaccination programs.
After that crisis of confidence in 1999, scientists and public health officials hustled to conduct research that moved the state of science from:
- “We have no evidence that thimerosal in vaccines causes any neurologic problem,”
to this position:
- “We have high-quality evidence that thimerosal in vaccines does not cause any neurologic problem.”
Indeed, when I looked at FDA’s thimerosal webpage recently, it listed more than two dozen publications that provided evidence of no hazard with thimerosal as a vaccine preservative. Various academic, pediatric, and global authorities post similar lists. No serious scientist disagrees.
As the 2000s progressed, thimerosal was accused—with scant evidence from dubious animal models—of triggering autism in young children. These accusations persist even after the removal of thimerosal from almost all childhood vaccines. Autism rates continued to increase after thimerosal was withdrawn—just the opposite of what would be expected if thimerosal caused autism.
There are no new data, neither pro nor con, related to thimerosal safety suddenly available. Society and the scientific research community has ample evidence that thimerosal as a preservative is not harmful to people and never was. Thimerosal is almost completely out of the vaccine inventory anyway. Ample alternative influenza vaccine dosage forms are available today to anyone who wants to both sidestep thimerosal and get the protection vaccines afford.
What can you do in your pharmacy?
Know the facts. Thimerosal was a big story in 1999. The important questions were asked and answered in the 2000s, which was public health’s duty to the American people. It is now settled science that thimerosal was never a hazard. For anyone to claim the contrary would require evidence-based, scientifically sound proof that withstands peer review.
And the individual patients in your pharmacy, how would you counsel them?
Remember all the lessons of good communication about vaccine benefits and risks from the COVID-19 vaccine rollout. The resources at vaccineconfident.pharmacist.com are still posted and available to you. While much of that content focuses on COVID-19 vaccines, the general principles apply to other vaccine-preventable diseases:
- Listen with respect
- Understand the basis for your patients’ concerns
- Ask permission to share information
- Recall that connections are more important than statistics
- Convey concern for their health
And if you have a patient so adamant they will not accept anything with thimerosal in it, just reach into the part of your refrigerator with the single-dose influenza vaccine containers and ask “Which arm (or nose)?”
Col (Ret) John D. Gräbenstein, RPh, PhD, FAPhA, is the managing editor of IZ Express. For more information, visit immunize.org.