Thimerosal: Preservative or poison? ACIP debates mercury-based vaccine ingredient

Lyn Redwood RN, MSN, FP, presents “Thimerosal as a Vaccine Preservative” during the June 26, 2025, meeting of the Advisory Committee on Immunization Practices. This image was taken from the meeting webcast.

The Advisory Committee on Immunization Practices (ACIP) met this week and voted to recommend vaccinations against influenza and respiratory syncytial virus (RSV).

Their votes to advise the U.S. Centers for Disease Control and Prevention, and the discussion of ACIP members and liaisons from medical organizations, included deliberation about an ingredient in some vaccines: thimerosal. It is a mercury-containing organic compound used since the 1930s as a preservative in a number of biological and drug products, including vaccines, to prevent the growth of microbes, according to the U.S. Food and Drug Administration (FDA).

The substance is no longer used in the United States for vaccines routinely recommended for children aged 6 years and younger, according to FDA. It remains used in some domestic flu vaccines, according to FDA, and speakers in the ACIP meeting said it remains in use abroad in vaccine multi-dose vials. A commentary from the University of Minnesota’s Vaccine Integrity Project stated: “Thimerosal has long been a focus of anti-vaccine activists and the genesis for attempts to connect vaccines with autism.”

Since his nomination, health experts have questioned the potential guidance and direction of Health and Human Services Secretary Robert F. Kennedy Jr. — who has publicly questioned the safety and efficacy of vaccines. The ACIP meeting was the first with new members appointed by Kennedy, who fired the prior 17 members. The new members are Chair Martin Kulldorff, MD, PhD; Joseph R. Hibbeln, MD; Retsef Levi, PhD; Robert W. Malone, MD; Cody Meissner, MD; James Pagano, MD; and Vicky Pebsworth, OP, PhD, RN.

Vaccine skeptic at CDC

This week, national news outlets reported on elements involving vaccination, ACIP, Kennedy, and the presentation in the second day of the two-day ACIP session.

CBS News reported Lyn Redwood RN, MSN, FP, apparently will be on staff in CDC’s vaccine safety office. Redwood is a former leader of Kennedy’s anti-vax group Children’s Health Defense.

Apart from the ACIP deliberations, Sen. Patty Murray (D-Washington) called for her to be fired.

“Vaccines work — they are safe, effective, and lifesaving,” Murray said in a statement. “We cannot allow a few truly deranged individuals to distort the plain truth and facts around vaccines so badly. I know that my Republican colleagues know this is wrong — now is not the time to be silent. Kids’ lives are on the line. Anti-vaccine conspiracy theorist Lyn Redwood has no place serving as a health advisor at CDC — or anywhere in the department.”

Global ramifications

Meanwhile, this week Kennedy took to social media site X, formerly Twitter, to publish a rebuttal to a Guardian report previewing the ACIP meeting. The Guardian reported “The preservative has been deemed safe,” but did not cite any peer-reviewed study. Instead, the news outlet fulfilled “the pharma-financed mainstream media’s mantric ritual of dutifully parroting the propaganda tropes spoon-fed them by vaccine makers and their captive regulators,” Kennedy wrote. He also used X to announce the United States would stop contributions to GAVI, The Vaccine Alliance, for neglecting vaccine safety. “When vaccine safety issues have come before GAVI, it has treated them not as a patient health problem, but as a public relations problem,” Kennedy wrote.

GAVI this week announced its Global Summit: Health and Prosperity through Immunisation, spurred more than $9 billion in financial commitments for its five-year strategic period from 2026 to 2030. The organization’s GAVI 6.0 plan aims to protect up to 500 million children from preventable disease, possibly averting up to 9 million future deaths and improving economic benefits for countries around the world.

Made-up information

CNN reported Redwood was scheduled on June 26 to give a presentation with made-up information. In that draft presentation published before the meeting, a slide cited a study about thimerosal that apparently did not exist. Author Robert F. Berman, PhD, a specialist in brain injury medicine, told CNN the presentation cited his name for a work that does not exist. A similar study he led “did not find evidence of thimerosal exposure at vaccine levels in mouse behaviors that we thought were relevant to autism.”

Mercury will not Make America Healthy Again

Redwood presented “Thimerosal as a Vaccine Preservative” during the ACIP meeting. Doctors and other clinicians did not contradict Redwood’s history of the vaccine additive, first patented in 1927 and marketed by Eli Lilly under the name Merthiolate in 1928.

“The fact that thimerosal from vaccines has been documented to raise blood mercury levels over known thresholds where developmental effects have been documented to occur during the first few months of life, means that particular ‘windows of vulnerability’ may have been breached,” the presentation said. “Even minor neurological impairment can have profound societal effects when amortized across the entire population and life span.

“Thimerosal can cross the placenta and blood brain barriers and converts to inorganic mercury in the brain at higher levels (than) methyl mercury,” the presentation said. “Removing a known neurotoxin from being injected into our most vulnerable populations is a good place to start with Making America Healthy Again.”

‘This is not a big issue’

No one asserted exposure to mercury is good for human health. But physicians and other clinicians pounced on the necessity of debating what many doctors and researchers consider settled science, and whether continued questioning will confuse the public and provoke fear of vaccines due to possible exposure to mercury.

“I’m not quite sure how to respond to this presentation. This is an old issue that has been addressed in the past,” Meissner said.

It is important to remember that thimerosal is metabolized into ethyl mercury, excreted from the body much more quickly than the methyl mercury found in fish and shellfish and associated with high neurotoxicity, Meissner said.

The United States has vaccine formulations for babies, young children, adolescents and adults that do not contain thimerosal. The additive is used in vaccines around the globe, but that is because single-dose vials are more expensive than multi-dose vials, and many countries cannot afford single-dose vials. Other nations consider ACIP recommendations, and removing thimerosal from vaccines used in other countries will increase costs and reduce access to vaccines, Meissner said.

The Food and Drug Administration decision to remove thimerosal from U.S. vaccines was not due to evidence of harm, but to reduce total exposure of mercury in the environment, which is reasonable, Meissner said.

“Of all the issues that I think we, ACIP needs to focus on, this is not a big issue,” Meissner said.

Possible harm?

Other members of ACIP were not so sure.

Pebsworth said other studies showed thimerosal has a higher binding affinity for tissue, and just because it left the blood, didn’t mean it exited the body. The finding about the blood was not a good predictor of toxicity because the brain, not the blood, was the organ of toxicity, she said.

Regarding third world countries, there are other approved preservatives that can be used in place of thimerosal, Pebsworth said.

“We have made such tremendous advances that we should not be relying on a product that is so neurotoxic when there's other safer preservatives available as well,” she said.

Meissner agreed there are other preservatives to use in vaccines. “But to make the industry jump through hoops for something for which there is no evidence of harm, I think is an issue that needs further discussion,” he said.

Retsef said ACIP should consider whether harm is from cumulative exposure and the committee should do what it can to control the controllable sources of exposure.

“We know that mercury is a toxin, so the key thing is to minimize the total exposure, the cumulative exposure. And we do that by removing exposure in any way we can. And there are some exposures we can't remove, but whatever we can remove would help reduce the total amount of exposure to mercury,” Retsef said.

Evidence or opinion?

Clinician liaisons from advisory organizations sounded far from convinced about a looming danger from thimerosal.

ACIP was taking time to discuss something with “very, very limited” potential for exposures because it was removed from a lot of products that reach children more than 20 years ago, said Stacy Buchanan, DNP, APRN, CPNP-PC, representing the National Association of Pediatric Nurse Practitioners. The FDA website showed it remains in use for only about three multi-dose products for influenza, she said.

Meissner’s point — that there was no evidence of harm from thimerosal — was worth emphasizing, said Matt Zahn, MD, representing the National Association of County & City Health Officials. Other countries use it in vaccines and there is a 25-year robust history of studies of children who have received those vaccines, he said.

“What are the psychological issues associated with broadcasting widely the notion that this question has not been answered or is a significant issue that CDC needs to dedicate public health resources to? I feel like it's a question been answered, and I feel like as (a pediatric) physician, as a public health practitioner, I'm very confident that thimerosal-containing vaccines are safe,” Zahn said.

There is overwhelming data that show prenatal exposure to thimerosal has not been linked to autism spectrum disorder, said Naima Joseph, MD, MPH, FACOG, representing the American College of Obstetricians and Gynecologists. Regarding flu, maternal immunization provides infant benefits against the risk of hospitalization that disrupts maternal infant bonding and can lead to development delays or infection, she said.

An internal medicine perspective

American College of Physicians President Jason Goldman, MD, MACP, said Meissner spoke eloquently about some of the concerns.

“This committee has always prided itself on openness, transparency, and review of data and evidence. So, I am wondering if we will have an actual scientific presentation with peer-reviewed literature, strong evidence, to actually discuss this issue, as many statements have been made here today without support of science or evidence, but merely opinion,” Goldman said. “So, in fairness, will we be seeing if necessary even though the established data for decades really speaks to the safety and efficacy of thimerosal, if necessary, will there be an actual CDC presentation done by staff scientists, physicians, and those who are subject matter experts with accurate, peer-reviewed scientific data for the ability for the committee to review, or will we have lay person presentations only?”

If there is a concern about thimerosal in vaccines, why is FDA not restricting its use in vaccines? said Molly Howell, an executive committee member of the Association of Immunization Managers.

Kelly Goode, PharmD, the liaison for the American Pharmacists Association, echoed Goldman’s question about the ACIP review process and transparency, and seeing the evidence for recommendations, including the balance of benefits and harms, the values and preferences of affected populations, the resource implications and equity considerations, before an ACIP vote.

Open to other viewpoints

Kulldorff said the committee had a presentation from Lynn Redwood, who is very knowledgeable about vaccines. This committee will always be open and receive comments from a variety of people as part of us being transparent. Earlier in the meeting, ACIP had comments today on other topics from the pharmaceutical companies, he said.

“That's the principle that we have, and I think it's inappropriate to dismiss a presentation just because the person does not have a PhD or an MD,” Kulldorff said. “There are a lot of knowledgeable people who we would like to hear from, and we want to hear from a variety of viewpoints. And I think today's discussion is a very good example that we have received input from a variety of people on this topic. We will continue to do that, as this committee moves forward with future meetings.”

Where are the pediatricians?

Kennedy, vaccine skeptics and advocates all agree children’s health is paramount in considering the medical treatments. But children’s health specialists did not participate in this week’s ACIP deliberations.

American Academy of Pediatrics President Sue Kressly, MD, FAAP, said ACIP’s process is “no longer credible,” with Kennedy firing the 17 former members.

The move was “childish,” Meissner said.

“I would like to comment on the decision by the American Academy of Pediatrics to boycott this meeting,” Meissner said. “In the past, the AAP has been very helpful generating recommendations. And I think it's somewhat childish for them not to appear.

“It's dialogue that leads to the best recommendations for the use of vaccines,” he said. “I mean, I'm led to think that they didn't want to come because they didn't want to have to support some of their decisions that are difficult to do. But in the future, I strongly recommend the American Academy of Pediatrics to participate in these meetings, as they always have.”