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A Centers for Disease Control and Prevention advisory panel vote to recommend against use of seasonal influenza vaccines containing small amounts of thimerosal followed a presentation that misled on the risks of the rarely used preservative.
There isn’t evidence that thimerosal in vaccines is harmful, and studies assessing a variety of health problems, including neurological conditions, have supported its safety.
Despite this, a June 26 presentation by longtime anti-vaccine advocate Lyn Redwood, given before the Advisory Committee on Immunization Practices, made unfounded or misleading claims, including that thimerosal is ineffective and a neurotoxin. (ACIP, which has been guiding the CDC’s vaccine recommendations since 1964, was recently completely reconstituted by Health and Human Services Secretary Robert F. Kennedy Jr., in an unprecedented move.)
Thimerosal, which is mercury-based, has long been a focus of anti-vaccine groups, including Children’s Health Defense, the nonprofit founded and formerly chaired by Kennedy. He has credited Redwood, a retired nurse practitioner who was also involved in the founding and past leadership of the nonprofit, as one of the people who introduced him 20 years ago to the thoroughly debunked claim that thimerosal in vaccines causes autism.
Thimerosal has been used in vaccines since the 1930s but in the U.S. today is only present in flu shots taken from multidose vials, as the preservative is needed to prevent the growth of germs that could be introduced each time a needle enters the vial. In 1999, the Food and Drug Administration asked that vaccine manufacturers stop offering thimerosal-containing versions of vaccines routinely given in infancy as a precautionary measure, even though there wasn’t evidence of harm. Since 2001, no vaccines for children have included thimerosal, except for some flu vaccines.
Subsequent evidence continued to support the safety of thimerosal-containing vaccines, and when flu vaccines were recommended annually for all children in 2004, ACIP endorsed them regardless of whether they contained the preservative. However, over the years Americans increasingly have gotten their flu vaccines in single-dose versions, which do not need preservatives. Flu vaccines with thimerosal made up less than 5% of vaccines given during the last flu season in the U.S., the FDA representative on the committee reported during the meeting.
Ordinarily, work groups prepare for months preceding an ACIP vote, and someone — often a CDC expert — publicly presents evidence on the topic at hand to the panel. These presentations contain not only information on benefits and harms but also on practical implications of the decision for public health. None of this happened prior to the thimerosal vote.
“There were many studies on the other side of the question that documented the safety of thimerosal that were not included, so it was a highly opinionated, data-sparse, incomplete presentation,” Dr. William Schaffner, a professor of infectious diseases at the Vanderbilt University Medical Center, told us, speaking of Redwood’s presentation. “And certainly in recent years, it would not have been permitted in that form.”
Before ACIP convened, a CDC evidence review on thimerosal-containing vaccines and neurodevelopmental outcomes that did include many of the studies demonstrating thimerosal’s safety was posted to the agency’s website for the meeting, but then removed. Dr. Robert Malone, a new member of the committee who has a history of misleading on vaccines, said during the meeting that the document “was not authorized by the Office of the Secretary.”
The “document by the CDC vaccine safety office did not go through the appropriate process to be posted,” an HHS spokesperson told us via email. “Nevertheless, our commitment remains the same: to evaluate the data and that’s what the new members did.” The spokesperson added that the “document was included in the ACIP member briefing packets.”
The newly reconstituted ACIP “used absolutely none of the usual aspects of deliberation or evidence” before making a recommendation, Dr. Paul Hunter, a family physician in Madison, Wisconsin, and an ACIP member between 2016 and 2020, told us. “It’s settled science that the benefits of the thimerosal-containing flu vaccines far outweigh the risks,” he added.
Misleading and One-Sided Presentation
Redwood, who has stated that she believes her son’s autism was caused by thimerosal in vaccines and helped originate the idea that thimerosal in vaccines causes autism, misleadingly cast thimerosal as a dangerous neurotoxin.
“Removing a known neurotoxin from being injected into our most vulnerable populations is a good place to start with Making America Healthy Again,” Redwood said, reading from her slides at the end of her presentation.
While at higher doses thimerosal can be harmful, there isn’t evidence that in the small amounts in vaccines thimerosal poses any safety risk, other than rare allergic reactions or temporary redness, swelling or itchiness around the injection site in some individuals.
The type of mercury in thimerosal is ethylmercury, which is substantially different and less toxic than methylmercury, which is what accumulates in fish from the environment. The 1999 decision to phase out thimerosal in children’s vaccines in the U.S. was based on calculations using guidelines on methylmercury exposure from the Environmental Protection Agency, but it’s clear the substances have different effects. As a 2006 review noted, a patient who consumed a massive dose of thimerosal became very ill, but completely recovered, in contrast to a methylmercury poisoning, which occurred at a much lower dose and left the individual with permanent brain damage. Similar findings have been reported in experiments on animals.
Numerous studies have evaluated the effects of thimerosal in vaccines and have not identified any neurological harms. (There are a few studies that link thimerosal-containing vaccines to tics, but only weakly. Other studies have not identified these associations, or have observed them inconsistently.) Moreover, the amount of mercury in a flu shot is about the same as what is in a 3 ounce portion of tuna, according to the FDA.
In 2004, the Institute of Medicine — an independent nonprofit now known as the National Academy of Medicine — concluded that “the evidence favors rejection of a causal relationship between thimerosal-containing vaccines and autism.” Credible studies conducted since then have also not identified any link, and indeed, as thimerosal has been removed from vaccines, autism diagnoses have gone up — not down.
Redwood avoided making direct claims about autism, but selectively presented evidence to paint thimerosal as problematic. For example, she cited studies of cells in petri dishes and pointed to a 2000 study in infants that showed a detectable rise in mercury in the blood following a dose of the hepatitis B vaccine.
She did not, however, present data showing harm in humans (during the discussion, she briefly referred to tics, but as we said, that is an inconsistent finding). Nor did she share information from other pharmacokinetics studies in babies, including premature ones, that found that the total blood mercury levels following vaccination remained low overall, and quickly declined, likely as babies excreted the ethylmercury in their feces.
Redwood also did not mention the Institute of Medicine’s 2004 findings, even as she repeatedly cited the group’s 2001 recommendation to use thimerosal-free vaccines.
“I’m not quite sure how to respond to this presentation. This is an old issue that has been addressed in the past,” Dr. Cody Meissner, a pediatrician at the Dartmouth Geisel School of Medicine, said during the meeting, going on to note the differences between ethyl- and methylmercury. “There is no scientific evidence that thimerosal has caused a problem.” Meissner was the sole ACIP member to vote against the recommendation to remove thimerosal from multidose seasonal flu vaccines.
Anders Hviid, head of the epidemiology research department at the Statens Serum Institut in Denmark, who has studied thimerosal-containing vaccines, told us in an email that it’s “easy to cherry pick results from lab and animal studies. It does not translate into harm to human children.” He added that studies from Denmark, the U.K. and the U.S. have looked for associations between “thimerosal-containing vaccines and autism and other neurodevelopmental outcomes and found no cause for concern.”
Redwood also emphasized a particular finding from a 2005 study of monkeys that were given either four thimerosal-containing vaccines in the first three weeks of life or fed an equivalent amount of methylmercury on the same schedule. Redwood did not share that the animals given methylmercury retained the mercury in their blood for much longer and had about three times as much total mercury in their brains than those that received vaccines. Instead, she highlighted the finding that vaccinated animals had more inorganic mercury — a type of mercury without carbon atoms — in their brains than those exposed to methylmercury.
Regardless, subsequent studies by some of the same authors provided reassurance that the thimerosal exposure did not harm the monkeys’ brains. A 2015 study looking at monkeys that were given thimerosal-containing vaccines found “no consistent evidence of neurodevelopmental deficits or aberrant behavior.”
Another study, published the same year, similarly identified no behavioral or neuropathological changes after giving monkeys the vaccines recommended for children in the 1990s. “This study does not support the hypothesis that thimerosal-containing vaccines and/or the MMR vaccine play a role in the etiology of autism,” the paper’s abstract concluded.
Notably, that study was partly funded by SafeMinds, an advocacy group that Redwood co-founded that has promoted the discredited idea that vaccines cause autism and has advocated the removal of thimerosal in all vaccines for decades. (A new ACIP member, Vicky Pebsworth, previously served on the group’s board of directors.)
Again, Redwood neglected to mention the findings of either study.
“It is important to note that concerns about the toxicity of thiomersal are theoretical and that there is no compelling scientific evidence of a safety problem related to its use in vaccines,” a World Health Organization webpage reads, using an alternate name for the preservative.
“Whenever you do the right studies — clinical studies looking at people who did or didn’t get thimerosal-containing vaccines — you don’t find any evidence for neurodevelopmental problems,” Dr. Paul A. Offit, a vaccine expert at the Children’s Hospital of Philadelphia, told us.
Dr. Michael Pichichero, vice president of the Rochester General Hospital Research Institute who performed several pharmacokinetic studies of thimerosal in the 2000s, was asked in 2012 to review the literature on the preservative since 2008 for the World Health Organization.
“No new evidence could be found in the published literature that brings into question the decision by WHO to endorse the continued use of thimerosal as a safe preservative in multi-dose vaccines,” he concluded in his presentation.
After a review of the evidence, the WHO’s vaccine safety committee concluded that “no additional studies of the safety of thiomersal in vaccines are warranted and that available evidence strongly supports the safety of the use of thiomersal as a preservative for inactivated vaccines. … Thiomersal allows millions of people worldwide to have access to life-saving vaccines and to date, no other safer and equally efficacious alternative has been identified for many vaccines.”
In an earlier commentary for Pediatric News, Pichichero explained that the initial trigger for concern about thimerosal — that the cumulative amount of mercury babies might receive from vaccines exceeded the EPA’s guideline for methylmercury — was misunderstood. The guideline, he wrote, was “actually meant to serve as a warning of methylmercury in the environment to trigger further investigation, not as a ‘safety threshold.’” Three other groups’ methylmercury guidelines were not exceeded, he added, and most critically, the guidelines applied to a different form of mercury.
Redwood also argued that thimerosal is ineffective, pointing to decades-old studies and case reports as well as FDA decisions on topical and over-the-counter versions of the chemical, which are no longer in use in the U.S.
Dorit Reiss, a vaccine law expert at University of California Law San Francisco, told us that the cited FDA materials are “not relevant to thimerosal in vaccines,” as they “are all clearly very specific to topical use.”
It’s true that in some cases thimerosal has failed to prevent contamination in vaccines, but that doesn’t mean that it doesn’t work overall.
“Thimerosal is not perfect,” Dr. Walter Orenstein, an emeritus professor at Emory University School of Medicine, told us. But, he added, “the exception doesn’t necessarily outweigh the rule. And there are data that support thimerosal’s effectiveness.” Orenstein was director of the U.S. immunization program when concerns about thimerosal were first raised and is an author of a paper Redwood cited, which recounts instances in 1982 in which thimerosal in multidose vials did not successfully prevent bacterial contamination.
As the FDA explains on its website, at concentrations found in vaccines, thimerosal “has been shown to be effective in clearing a broad spectrum of pathogens” and “meets the requirements for a preservative as set forth by the United States Pharmacopeia.”
“The risk of contamination of vaccines cannot be completely eliminated even with the use of preservatives,” the webpage continues. “The literature contains several reports of bacterial contamination of vaccines despite the presence of a preservative, emphasizing the need for meticulous attention to technique in withdrawing vaccines from multi-dose vials. The need for preservatives in multi-dose vials of vaccines is nonetheless clear.”
It’s also worth noting that when Redwood’s slides were first posted to the CDC website, she included two references to animal studies that do not exist to argue against including thimerosal in vaccines.
In one case, she claimed a 2008 paper identified long-term “consequences” in the brains of rats following thimerosal exposure. But the lead author told CNN and Reuters that he did not have a paper by the listed name. A similar-sounding study he published the same year, conducted in a certain strain of mice, largely failed to find evidence that thimerosal in vaccines altered animal behavior.
Implications of Recommending Against Thimerosal
Vaccine and policy experts told us that while the direct effect of the vote on access to flu vaccines would be small, it could make the vaccines more difficult to access for some people.
Only 4% of flu vaccines contained thimerosal during the 2024-2025 season, according to the CDC document that was removed, including just 2% of the vaccines provided via government programs to uninsured or underinsured kids and adults.
ACIP recommendations help guide which vaccines insurers are required to cover at no cost and which vaccines are covered under government programs.
“This change in recommendation, assuming CDC (or HHS) supports it, means that in most cases, payers will not be required to cover this vaccine at no-cost,” Jennifer Kates, senior vice president and director of the Global Health & HIV Policy Program at KFF, told us via email. “Given that these account for a small share of the flu vaccine market and there are alternatives, payers may just decide not to cover it, though they certainly can choose to.”
The fate of thimerosal-containing flu vaccines in the U.S. will also depend on the decisions of vaccine makers. According to a list on the FDA website, two companies, CSL Seqirus and Sanofi, produce approved multidose seasonal flu vaccines for the U.S. market. Spokespeople from both companies told us via email that they did not anticipate supply problems as a result of the new recommendation, and the CSL Seqirus spokesperson said the company was “committed to supporting our customers in fully transitioning to single-dose syringes.”
Still, some experts expressed concern that the change in recommendations could have a significant effect for some populations.
Hunter, who is also a former vaccine science fellow for the American Academy of Family Physicians, told us that thimerosal-containing multidose vials are most useful in a setting with low resources and a high volume of vaccinations, like a mass clinic.
The thimerosal-containing vaccines are less expensive per dose and require less refrigerator space per dose than single-dose vaccines, Hunter explained. “I don’t think it’s going to be a huge issue, but it will be an issue in some places for sure,” he said.
Multidose vials are more commonly used outside the U.S. Hviid said the vote could have a “spill-over effect into countries relying more on multi-dose vials which could increase costs and reduce access to vaccines.”
Dr. Joseph Hibbeln, a retired psychiatrist and neuroscientist and new ACIP member, said following the vote that “there is a significant benefit to the use of multidose vaccines instead of single-dose vaccines” and suggested ACIP in the future consider multidose vials with “other, better preservatives.”
Redwood had previously said that “other safer preservatives” are available.
But vaccine manufacturers can’t simply decide to replace thimerosal with a different preservative. As the WHO has noted, different preservatives can interact with other vaccine ingredients in “unpredictable” ways, potentially altering a vaccine’s safety, efficacy and quality.
“If you go to a new preservative you’ll have to do all kinds of studies to show that they’re safe,” Schaffner told us.
Meissner said in response to Redwood’s claim that the other preservatives “have not been studied to the same extent” as thimerosal. There are only three other preservatives that appear in FDA-approved vaccines, often in vaccines that are not recommended for routine use.
Dr. Thomas Moore, an infectious disease physician and clinical professor of medicine at the University of Kansas School of Medicine in Wichita, told us that even in a hypothetical scenario in which a company did the needed work to use a different preservative in flu vaccines, it might make the vaccines more costly.
Biostatistician and epidemiologist Martin Kulldorff, ACIP’s new chair, argued during the meeting that removing thimerosal from vaccines could increase vaccine confidence. “I’m not an expert on marketing, but if you put mercury in a product, people are not going to want to buy those products,” he said.
But experts we interviewed expressed concern that removing recommendations for vaccines without evidence of harm would mislead people about vaccine safety.
KFF’s Kates said that the deliberation “could fuel increased vaccine hesitancy and further drive down vaccination rates in the U.S. and elsewhere,” despite a lack of new data “presented or vetted by the CDC on safety issues.”
Moore said that thimerosal is being used “as a cudgel to destroy confidence in vaccines in general,” despite there being “no evidence of any harm” from thimerosal-containing vaccines.
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