On June 26, 2025, the US Advisory Committee on Immunization Practices (ACIP) voted to stop recommending flu vaccines containing thimerosal, a mercury-based preservative that has long been the subject of controversy. This decision, made by a newly reconstituted ACIP under the leadership of Health and Human Services Secretary Robert F. Kennedy Jr., has sparked intense debate among public health officials, scientists, and vaccine advocates.
The ACIP, which advises the Centers for Disease Control and Prevention (CDC) on vaccine policy, has been a cornerstone of US immunization guidelines since its establishment in 1964. However, in early June 2025, Secretary Kennedy abruptly dismissed all 17 previous ACIP members, citing conflicts of interest, and appointed eight new members, some of whom have expressed skepticism about vaccines. This shake-up has raised alarm bells across the medical community.
"A clean sweep is necessary to reestablish public confidence in vaccine science," Kennedy stated when announcing the overhaul. Yet many experts see the move as reckless. Amesh Adalja of Johns Hopkins University remarked, "I had confidence in the ACIP before Kennedy did [this]. Now I have zero. Unilaterally removing an entire panel of experts is reckless, shortsighted and severely harmful." Similarly, Tina Tan, president of the Infectious Diseases Society of America, called the action "reckless, shortsighted and severely harmful." The new panel, which met for the first time on June 26, wasted no time in issuing controversial recommendations.
One of the most contentious votes was to stop recommending flu vaccines containing thimerosal. Thimerosal, also known as thiomersal, is a preservative used since the 1930s in some multi-dose vaccine vials to prevent bacterial contamination. It metabolizes into ethylmercury in the body, a compound distinct from the highly toxic methylmercury found in certain environmental sources like fish. Numerous studies have shown that ethylmercury is cleared from the bloodstream within 30 days, even in infants, and is not associated with toxicity.
Despite this, Secretary Kennedy has described thimerosal as "toxic" and carcinogenic, claims that have gained traction online but are widely disputed by scientists. Kennedy, who founded the anti-vaccine group Children’s Health Defense, has pushed for the complete removal of thimerosal from vaccines, reigniting a debate that had largely faded from public health discussions decades ago.
During the ACIP meeting, a presentation advocating for the removal of thimerosal was delivered by a former leader of Kennedy’s group, who notably lacks a medical or scientific degree. This approach contrasts sharply with past procedures, where vaccine-related topics were reviewed extensively by expert work groups over months before being brought to a vote. Moreover, a CDC report supporting the safety of thimerosal, prepared before the meeting, was mysteriously removed from the agency’s website shortly after the vote.
Dr. Cody Meissner, a pediatrician and the sole ACIP member to vote against the recommendation, emphasized the risks of influenza far outweigh any theoretical concerns about thimerosal. "The risk from influenza is so much greater than the nonexistent, as far as I know, risk from thimerosal," he told The Washington Post.
Public health experts warn that the decision could reduce vaccine availability in some regions without providing any real safety benefits. Thimerosal has not been used in most flu vaccines for years, and only a small fraction of vaccines administered during the 2024-2025 flu season contained it—about 3% of children and 2% of adults aged 65 and older. Most vaccines today come in single-dose vials or syringes that do not require preservatives like thimerosal.
The controversy surrounding thimerosal dates back to the late 1990s, when fears about mercury exposure led the American Academy of Pediatrics and the U.S. Public Health Service to recommend its removal as a precautionary measure. However, this was never due to evidence of harm. A 1999 FDA review found no side effects from thimerosal aside from rare allergic reactions. Further, a 2004 Institute of Medicine review of over 200 scientific studies found no causal link between thimerosal-containing vaccines and autism or neurodevelopmental disorders.
These fears were exacerbated by a discredited 1998 study published in The Lancet by Andrew Wakefield, which falsely linked the MMR vaccine to autism. The MMR vaccine never contained thimerosal, and Wakefield’s study was later retracted, with the author stripped of his medical license for ethical violations and data falsification. Nevertheless, misinformation about thimerosal’s safety has persisted, fueled by anti-vaccine groups and amplified by social media.
Internationally, other countries have taken precautionary steps regarding thimerosal without evidence of harm. The UK removed thimerosal from routine immunizations between 2003 and 2005, and the European Medicines Agency has reaffirmed its safety multiple times, most recently in 2016. The World Health Organization continues to affirm that there is no evidence of toxicity from thimerosal-containing vaccines in infants, children, or adults.
Aside from the thimerosal vote, the newly formed ACIP panel has also announced plans to review the entire childhood vaccine schedule and reconsider recommendations for the hepatitis B vaccine. In May 2025, Secretary Kennedy unilaterally announced that COVID-19 vaccines would no longer be recommended for most children and pregnant individuals, a decision made without ACIP input and breaking decades of precedent. The CDC subsequently updated its guidelines to suggest COVID-19 vaccination decisions for children be made in consultation with healthcare providers and based on personal preference.
Critics argue these shifts undermine the credibility of vaccine policy in the United States. Dr. David Margolius, director of public health in Cleveland, warned that Kennedy’s actions have "opened the door for public health problems that have been dealt with decades ago to come back into the picture." Meanwhile, some new ACIP members, such as Robert Malone, who helped develop mRNA vaccine technology but has voiced controversial views about COVID-19 vaccines causing heart attacks and strokes, have further fueled concern. Although a 2024 study of nearly 46 million adults found COVID-19 vaccination was associated with lower rates of heart attacks and strokes, it also noted rare increased incidences of myocarditis and pericarditis, mainly in younger males.
A CDC spokesperson defended Malone, stating he "believes in rigorous, evidence-based review, not blind endorsement," and that questioning policies does not equate to being anti-vaccine. Nevertheless, many experts feel the new panel lacks sufficient expertise in vaccines, infectious diseases, and epidemiology, rendering the ACIP less relevant.
Despite these upheavals, public health officials emphasize that vaccine access is unlikely to be significantly affected. Most seasonal flu vaccines do not contain thimerosal, and the CDC maintains that COVID-19 vaccines remain available for those who choose them. Individuals seeking to avoid thimerosal can request flu vaccines supplied in single-dose vials or syringes.
The debate over thimerosal highlights the ongoing tensions between scientific consensus and vaccine skepticism in the US. While the ACIP’s recent decisions reflect a shift in vaccine policy under Secretary Kennedy’s leadership, they have also sparked fears of eroding public trust and complicating efforts to protect communities from preventable diseases.
As the ACIP continues its review of vaccine recommendations, the nation watches closely, weighing the balance between precaution, evidence, and public health priorities in an era marked by misinformation and deep divisions over vaccines.
