Thursday, October 2, 2025

Preservative from old flu-shot debate

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Introduction to Thimerosal

The Trump administration’s vaccine advisers are bringing up an old flu-shot debate: whether it’s time to wipe out the last small fraction of those vaccines that contain a controversial preservative called thimerosal. It’s a question seemingly laid to rest years ago, as studies showed no evidence that the preservative causes any health problems. “This is really a nonissue,” said Dr. Sean O’Leary of the American Academy of Pediatrics.

Background on Thimerosal

Thimerosal is a preservative used in certain vaccines since the 1930s, as well as in some other medical products. It was mostly used in multi-dose vials of vaccine, to prevent bacterial contamination as the vessel was repeatedly punctured to withdraw a dose.

What is Thimerosal?

Thimerosal contains a form of mercury, which raised questions in the late 1990s. However, it’s not the same as the toxic type found in some seafood, called methylmercury. Instead, it’s a different type called ethylmercury that the body can excrete, O’Leary explained. The amount of ethylmercury per vaccine dose was small, and studies found no evidence of harm.

Why the Concern?

Nor was thimerosal used in all vaccines. For example, vaccines for chickenpox, polio, and measles, mumps, and rubella never contained it, according to the Centers for Disease Control and Prevention. But as a precaution, the U.S. phased the preservative out of childhood vaccines. Since 2001, all vaccines routinely recommended for children age 6 and younger in the U.S. come in formulas that don’t contain thimerosal.

Current Use of Thimerosal

The exception is a small subset of flu shot formulas in multi-dose vials that could be used for adults or kids. The vast majority of children, however, get their flu vaccination from a single-dose shot instead, O’Leary said. According to the CDC, 96% of all flu vaccines in the U.S. administered last fall and winter — and an even higher share of those used in federally funded programs — were thimerosal-free.

Debate Renewal

U.S. Health Secretary Robert F. Kennedy Jr. is a longtime antivaccine activist, and he has long contended there was a tie between thimerosal and autism. On Thursday, his newly appointed vaccine advisers were set to hear a presentation from Lyn Redwood, a nurse practitioner who once ran the antivaccine group that Kennedy founded. She will suggest that the remaining thimerosal-containing flu shots should be removed, according to meeting materials posted earlier this week. Among the claims were that they could be a risk to pregnant women. A new CDC staff analysis prepared for the meeting again showed no link between the preservative and autism or any other neurodevelopmental disorders.

Expert Perspectives

Some experts note that autism rates rose after thimerosal was removed from young children’s vaccines in the U.S. The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education and the Robert Wood Johnson Foundation. The AP is solely responsible for all content.

Originally Published: June 26, 2025 at 2:27 PM EDT

Conclusion

The debate over thimerosal in flu vaccines has been rekindled despite extensive research showing no link between the preservative and health issues such as autism. The scientific consensus supports the safety of vaccines, whether they contain thimerosal or not.

FAQs

  • Q: What is thimerosal?
    • A: Thimerosal is a preservative used in some vaccines and medical products to prevent bacterial contamination.
  • Q: Why was thimerosal controversial?
    • A: Thimerosal contains a form of mercury, which raised health concerns, although it is a different, less toxic form than found in some seafood.
  • Q: Is thimerosal still used in vaccines?
    • A: Yes, but only in a small subset of flu shot formulas in multi-dose vials, mostly for adults or kids. The majority of flu vaccines are thimerosal-free.
  • Q: What do studies say about thimerosal and health risks?
    • A: Studies have found no evidence that thimerosal causes health problems, including autism or other neurodevelopmental disorders.
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