Something changed on a Wednesday in late July. Health and Human Services Secretary Robert F. Kennedy Jr. put his signature on a document that vaccine critics had pursued for decades. Some cheered. Others warned of consequences no one could predict.
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A preservative called thimerosal would disappear from America’s flu shots. All of them. Kennedy called it a victory for children. Scientists called it something else entirely.
Between those two reactions lies a story about how science, politics, and public fear collided in a way that will ripple across the globe.
Kennedy Signs Policy After Reconstituting Federal Panel
Kennedy made his move official on July 23, 2025, when he adopted recommendations from the Centers for Disease Control and Prevention’s Advisory Committee on Immunization Practices. ACIP had voted 5-1 with one abstention on June 25-26 to remove thimerosal from all influenza vaccines distributed in the United States.
But that vote came only after Kennedy fired all 17 existing ACIP members and replaced them with eight hand-picked allies. Among the new appointees sat Vicky Pebsworth from the National Vaccine Information Center, an organization known for amplifying vaccine risks while minimizing benefits. Robert Malone, a vaccinologist involved in early mRNA research who later became a vaccine skeptic, also joined the panel.
Kennedy framed the decision as keeping a promise made 26 years ago. “After more than two decades of delay, this action fulfills a long-overdue promise to protect our most vulnerable populations from unnecessary mercury exposure,” he said in the official statement.
Policy change affects roughly 4 to 5 percent of current flu vaccine supply. Multidose formulations of Sanofi’s Fluzone and two shots from CSL Seqirus contain thimerosal. Single-dose prefilled syringes do not.
What Scientists Know About Thimerosal
Ethylmercury-based preservatives have prevented bacterial contamination in multidose vaccine vials since the 1930s. Thimerosal does one job and does it well. When medical providers puncture a vial multiple times to draw individual doses, bacteria can enter. Thimerosal stops that contamination before it starts.
Medical providers already removed thimerosal from most childhood vaccines in 2001. That decision came after the U.S. Public Health Service and the American Academy of Pediatrics asked manufacturers to phase out the preservative. Their request was precautionary, not based on evidence of harm.
Chemistry matters here. Thimerosal contains ethylmercury, which differs from methylmercury found in fish. Ethylmercury clears from the body faster than its dietary cousin. A flu vaccine contains about 25 micrograms of ethylmercury. A three-ounce serving of canned tuna contains roughly 40 micrograms of methylmercury.
More than 40 studies conducted over several decades found no link between thimerosal and developmental delays. Zero. Research showed no connection to autism, no tie to neurological problems, nothing that would justify removal on safety grounds.
World Health Organization vaccine chief Kate O’Brien stated her position after ACIP’s June vote. “There is no evidence of harm from the use of thimerosal,” she told reporters. “For some of the vaccine supply, this is a very important ingredient in order to assure that we have the doses that are needed to protect children from serious and life-threatening diseases.”
How Kennedy Shaped the Vote
Lyn Redwood presented the case for thimerosal removal at the June ACIP meeting. Redwood once led the World Mercury Project, the organization that preceded Children’s Health Defense. Kennedy founded and previously chaired Children’s Health Defense, an organization that produces anti-vaccine content.
Redwood appeared before ACIP as a private citizen, a departure from normal procedures. Her presentation avoided the scientific vetting process that CDC researchers typically conduct on ACIP materials.
Reuters discovered something odd in the days before Redwood’s presentation. One study cited in her materials did not exist. Redwood updated her presentation after the error came to light.
CDC scientists prepared their own presentation about thimerosal safety. Kennedy’s office pulled it from the agenda before ACIP members could review it. Robert Malone confirmed this during the meeting when other participants asked why the CDC document had vanished from the website.
Kennedy himself weighed in on social media the night before the vote. He posted detailed arguments about what he views as thimerosal risks, countering media coverage that questioned the scientific basis for removal.
One Doctor Voted No Three Times
Cody Meissner cast the sole dissenting vote across all three recommendations. Each time ACIP voted on whether children, pregnant women, or adults should receive only thimerosal-free flu vaccines, Meissner voted no.
A professor of pediatrics at Dartmouth Geisel School of Medicine, Meissner has served on both the CDC and the FDA vaccine advisory committees. He knows how these panels work and what standards they typically apply.
Meissner reminded his fellow members that ACIP generally bases recommendations on scientific evidence. The FDA has not removed thimerosal because no evidence supports that action. “No study has ever indicated any harm from thimerosal,” he said during the meeting.
He worried about practical consequences. Discouraging multidose vials could increase vaccination costs and limit access for some populations. What happens when someone walks into a clinic during flu season and only multidose formulations are available? “My point is, the risk from influenza is so much greater than the nonexistent, as far as we know, risk from thimerosal, so I would hate for a person not to receive the influenza vaccine,” Meissner explained.
Countries outside the United States rely more heavily on multidose formulations. Lower costs make these vials attractive for nations with limited health budgets. Meissner raised concerns about the message the ACIP’s recommendation would send to those countries.
Vicky Pebsworth abstained from all three votes, but not because she opposed removal. She objected to how the voting question was worded. Her abstentions meant nothing for the outcome.
Medical Community Pushes Back Hard
American Academy of Pediatrics reversed its earlier stance on thimerosal. In a June 2025 statement, AAP declared that “extensive research proves that thimerosal is a safe ingredient in vaccines, and it does not cause neurological problems or autism.” AAP went further, warning that “banning vaccine ingredients without solid scientific reasons sets a dangerous precedent and ultimately makes children less safe.”
AAP pointed to an inconvenient fact. Autism rates rose after manufacturers removed thimerosal from childhood vaccines in 2001. If thimerosal caused autism, rates should have dropped. They did not.
Dr. Sean O’Leary works as a pediatrician at Children’s Hospital Colorado. He previously served as a liaison to ACIP for the American Academy of Pediatrics. O’Leary expressed confusion about why thimerosal appeared on the meeting agenda at all. “I actually don’t know any pediatric practices that even use that multidose influenza vaccine,” he said.
Former ACIP liaisons raised alarms about implications for pandemic flu vaccines. During a pandemic, manufacturers typically produce vaccines in multidose formulations first. Those vials contain thimerosal. Steering people away from thimerosal-containing vaccines could hamper pandemic response efforts when speed matters most.
Medical experts noted how the decision embeds anti-vaccine talking points into federal policy. Arguments that have circulated for decades among vaccine skeptics now carry the weight of official government recommendations.
Anti-Vaccine Movement Celebrates
Children’s Health Defense and similar organizations spent years campaigning against thimerosal. Kennedy published a book before becoming HHS Secretary that linked the preservative to developmental disorders despite mountains of evidence to the contrary.
Groups that have promoted vaccine hesitancy for decades view Kennedy’s decision as vindication. Social media lit up with celebrations from activists who fought to remove thimerosal. Some posts claimed victory over corrupt scientists. Others suggested this was just the beginning.
Kennedy’s reconstituted ACIP will examine more vaccine components. He established two new working groups at the June meeting. One will evaluate the cumulative effects of the childhood vaccine schedule. Another will review vaccines that have been in use for more than seven years.
Targets include hepatitis B vaccination given at birth and the MMRV vaccine administered to children under 47 months. Both represent priorities for vaccine-skeptic activists.
Chair Martin Kulldorff, a statistician and epidemiologist formerly at Harvard University, outlined plans to address the MMRV vaccine at a future meeting. MMRV carries a slightly higher risk of febrile seizures when given as a first dose compared with separate MMR and varicella vaccines.
What Changes for Vaccine Supply
Manufacturers confirmed they can meet demand for single-dose flu vaccines. The Vaccines for Children program will not face interruptions. Adult vaccine supplies remain stable according to HHS announcements.
About 96 percent of flu vaccines distributed during the 2024-2025 season already came in thimerosal-free formulations. Changes for the 2025-2026 season will eliminate the remaining 4 percent.
Pharmacists already administer mostly single-dose prefilled syringes. Dr. Adam James noted in an interview that pharmacy patients rarely encounter thimerosal-containing vaccines. “Almost every vaccine we administer in pharmacies is a single-dose, prefilled syringe, meaning it doesn’t contain thimerosal,” he explained.
Kennedy approved all recommendations from ACIP’s April meeting. Other proposals from the June session remain under review. ACIP voted to recommend clesrovimab, an RSV monoclonal antibody, for infants. Members reaffirmed annual flu vaccination for people aged 6 months and older. FluMist self-administration and expanded age ranges for Flublok also received approval.
Which June recommendations Kennedy will sign and which he will reject remains unclear.
Global Health Officials Watch with Concern
Kennedy urged international authorities to follow America’s lead. “With the U.S. now removing mercury from all vaccines, we urge global health authorities to follow this prudent example for the protection of children worldwide,” he said in the official statement.
WHO officials pushed back against that suggestion. Kate O’Brien defended thimerosal as necessary for vaccine programs in many countries. Lower-income nations depend on multidose vials because they cost less than single-dose alternatives.
Removing thimerosal could complicate vaccine distribution in places where cold chain infrastructure is limited. Multidose vials make sense when transporting vaccines to remote areas. Single-dose syringes take up more space and require more careful handling.
Pressure from the United States might force other countries to eliminate a safe preservative. That could reduce vaccine access in regions where infectious diseases kill more children than in wealthy nations.
Manufacturers face new complications in the global market. Producing separate formulations for different countries increases costs and complexity. Some companies might choose to stop making multidose vials entirely, limiting options for countries that need them most.
Scientists worry Kennedy’s decision will embolden vaccine skeptics worldwide. When the United States bans an ingredient that decades of research have deemed safe, other nations face political pressure to do the same, regardless of scientific evidence.
Kennedy has more than thimerosal in his sights. Working groups he established will scrutinize vaccines that have protected children for decades. What gets reviewed next could reshape vaccine policy in ways that extend far beyond preservatives.
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