A major shift may be coming in the way flu vaccines are administered in the U.S. A federal advisory panel has recommended that flu shots given to children, adults, and pregnant women should no longer contain thimerosal—a mercury-based preservative. Although the ingredient has already been phased out of most vaccines for nearly 20 years, the Centers for Disease Control and Prevention (CDC) could soon formalize a nationwide recommendation to avoid it altogether.
This recommendation came during the first meeting of the newly restructured Advisory Committee on Immunization Practices (ACIP), now composed entirely of members appointed by Health Secretary Robert F. Kennedy Jr., a longtime vaccine skeptic.
The vote was not unanimous. One committee member, Dr. Cody Meissner of Dartmouth College, opposed the recommendation, arguing it targeted a non-issue. “The risk from influenza is so much greater than the non-existent, as far as we know, risk from thimerosal,” he said.
Still, many are asking: what exactly is thimerosal, and what does its removal mean for flu vaccinations?
1. Thimerosal Is a Mercury-Based Preservative
Thimerosal is a compound that contains about 50% ethylmercury. It’s been used in vaccines since the 1930s to prevent bacterial and fungal contamination in multi-dose vials. When healthcare providers draw multiple doses from the same vial, thimerosal helps keep the remaining vaccine safe for the next patient.
Today, most flu vaccines are provided in prefilled, single-use syringes that don’t require thimerosal. But in some settings—such as public clinics or areas with limited refrigeration—multi-dose vials remain useful because they’re more affordable and take up less space.
According to the CDC, 94% of the flu vaccines used in the 2024–25 season were already free of or had significantly reduced amounts of thimerosal.
2. Thimerosal Was Removed from Childhood Vaccines Over 20 Years Ago
Concerns over mercury in medicine surfaced in the late 1990s. In 1997, Congress passed the FDA Modernization Act, which required a review of mercury content in medicines, including vaccines.
While all forms of mercury can sound alarming, not all are the same. Methylmercury, found in fish, can build up in the body and is known to be harmful. Thimerosal contains ethylmercury, which is broken down and cleared by the body quickly, making it less of a concern.
Despite no proven harm, thimerosal was removed from most childhood vaccines by 2001 as a precautionary measure. Even then, health experts emphasized that there was no scientific evidence linking the preservative to health issues.
3. No Proven Link Between Thimerosal and Autism
The idea that vaccines cause autism has been widely discredited. It originated with a flawed 1998 study focused on the MMR (measles, mumps, rubella) vaccine. That study was later retracted, and its author, Dr. Andrew Wakefield, lost his medical license.
Still, fears lingered—spreading even to ingredients like thimerosal. However, multiple studies have since shown no connection between thimerosal and autism. A 2004 report by the National Academies found no such link, and follow-up studies in 2006, 2007, 2009, and 2010 confirmed the same.
In fact, autism diagnoses have continued to rise even after thimerosal was removed from most vaccines. As Dr. Jesse Goodman, former chief scientist at the FDA, pointed out, “That certainly would not be consistent with having anything to do with childhood vaccines.”
4. Most People Won’t Notice a Change if Thimerosal Is Phased Out
Since nearly all flu vaccines are already thimerosal-free, most people will not see any real change if the CDC officially recommends avoiding it. However, vaccine manufacturers and health centers may have to make adjustments.
Companies like Sanofi and Seqirus, which still produce limited quantities of flu vaccines containing thimerosal in multi-dose vials, have said they’re ready to transition to single-dose options without impacting supply.
“Only a very small number” of Sanofi’s flu shots contain thimerosal, the company said. Seqirus echoed that the preservative is used in a “very small proportion” of their vaccines and promised no delays in shipments.
Experts Warn Against Politicizing Vaccine Decisions
While the change may have minimal impact on most patients, public health experts are raising concerns about how the decision was made. Dr. Michelle Fiscus, chief clinical officer at the Association of Immunization Managers, criticized the vote, warning that it appears to disregard established scientific evidence and long-standing procedures.
“Is this now going to be the standard?” she asked. “That’s very concerning if that’s where this is heading.”
Dr. Goodman agrees, adding that politicizing vaccine safety could have broader consequences. “Flu vaccines aren’t perfect, but they save lives,” he said. “If there’s more difficulty in accessing them or affording them, at the end of the day, people would get hurt.”
Conclusion
Thimerosal has been largely absent from vaccines for nearly two decades. Multiple scientific reviews have shown it to be safe, with no connection to autism or other health issues. Yet, it continues to stir public debate—now elevated by political changes in vaccine policy leadership.
If the CDC officially accepts ACIP’s recommendation, the transition will be mostly administrative. But how these decisions are made—and whether they follow science—could influence public trust in vaccines far more than thimerosal ever did.
Health Tip:
Flu vaccines are most effective when taken early in the season. If you’re unsure which version you’re receiving, ask your provider—but know that both single-dose and multi-dose versions are safe, effective, and help protect you and those around you.
