The federal government could soon recommend that people only receive flu shots made without an ingredient called thimerosal.
The preservative has been absent from the majority of flu vaccines for nearly two decades, but was on the agenda of a committee that advises the Centers for Disease Control and Prevention on vaccine policy.
The June meeting was the first since earlier in the month, when Health Secretary Robert F. Kennedy Jr. sacked all 17 previous members of the Advisory Committee on Immunization Practices, or ACIP, and installed his own slate of seven.
Thimerosal has been a target of groups that question vaccine safety. That's despite "a long record of safe and effective use" as a vaccine preservative, according to the Food and Drug Administration.
In three separate votes, ACIP recommended that children, pregnant women and all adults receive single-dose flu immunizations with vaccines that don't contain thimerosal. The next step would be a decision by Kennedy or the CDC on the recommendations.
"It's sort of like they turned this meeting into solving a problem that doesn't really exist anymore," says Dr. Jesse Goodman, a former chief scientist at FDA who is now at Georgetown University.
The lone committee member to vote against the recommendations was Dr. Cody Meissner, a professor of pediatrics at Dartmouth College.
"Of all the issues that ACIP needs to focus on, this is not a big issue," he said. "The risk from influenza is so much greater than the non-existent, as far as we know, risk from thimerosal."
He added: "There is no scientific evidence that thimerosal has caused a problem."
But what is thimerosal and what would happen if it is effectively banned from flu shots? Here are four things to know.
1. It's a preservative that contains mercury
Thimerosal is a chemical compound that's about 50% mercury by weight, according to the FDA, and has been used in trace amounts as a preservative in vaccines and medicines since the 1930s.
The compound is used in vials containing multiple doses of the flu vaccine. Its job is to keep the vial from getting contaminated between patients and potentially getting someone sick, says Dr. Michelle Fiscus, chief clinical officer of the Association of Immunization Managers.
"If you're constantly going in and out of the vial to draw up a new dose, you run the risk of getting bacteria or fungus into that vial," says Fiscus. "And we don't want those vaccines to be contaminated."
She says although most flu vaccines come in single-dose prefilled syringes today, the multi-use vials are cheaper and take up less space in refrigerators. So for some health centers, they're more practical.
"When we're coming into flu season, it's not uncommon to get all of your flu vaccine upfront at the beginning of the season. And sometimes storage in the refrigerator can be an issue," Fiscus says.
Still, they're not common. According to the CDC, 94% of flu shots in the 2024-25 season were thimerosal-free or thimerosal-reduced.
2. Thimerosal has been used infrequently since 2001
In 1997, the FDA Modernization Act required the government to evaluate mercury in childhood vaccines.
But the heavy metal comes in different forms. Methylmercury is the compound found in seafood that doesn't break down easily and can be toxic. Ethylmercury, the form in thimerosal, has a different risk profile.
"It didn't distinguish between that kind of mercury and ethylmercury, which is a form of mercury that's very quickly managed by the body and eliminated through the kidneys," says Fiscus. "And ethylmercury is where thimerosal is derived."
Thimerosal was removed from most childhood vaccines in 2001, even though there was no evidence showing it caused harm to vaccine recipients, Fiscus says.
3. It has a discredited link to autism
Concerns that vaccines cause autism originated with a now-retracted 1998 study by Dr. Andrew Wakefield, which focused on the MMR vaccine against measles, mumps and rubella. Wakefield lost his medical license and The BMJ, a prominent medical journal, called it an "elaborate fraud" in a 2011 editorial.
Despite that, speculation about a link between vaccines and autism — once championed by Kennedy himself — continued and expanded to include thimerosal, even though a link to autism has been repeatedly disproven.
A 2004 report published by the National Academies of Sciences Engineering and Medicine reiterated that thimerosal wasn't associated with autism. Subsequent studies in 2006, 2007, 2009 and 2010 showed thimerosal was not associated with autism or neurophysiological problems, according to the CDC.
What's more, Georgetown's Goodman points out that despite thimerosal's removal from most vaccines, autism rates have continued to rise over the last 20 years, "which certainly would not be consistent with having anything to do with childhood vaccines."
4. Most people won't notice if thimerosal is no longer recommended in shots
Since most flu shots are administered with single-dose syringes, most people probably won't notice this fall and winter if the CDC decides to effectively ban thimerosal, Fiscus says.
Still, a thimerosal ban could present some challenges because health centers begin ordering flu vaccines in February for the next flu season. It's possible manufacturers will have to swap them out.
Sanofi, which makes multidose vials of a vaccine called Fluzone as well as single-dose syringes, says only a "very small number" of its doses contain thimerosal. "We acknowledge the recommendation of the new ACIP. We now await the decision by the CDC on the path forward. We will have sufficient supply of Sanofi flu vaccine to support customer preference for this season."
Seqirus, the other company that makes multidose vials of flu vaccine containing thimerosal, said in a statement to NPR that they "represent a very small proportion" of its total vaccine supply. "We are committed to supporting our customers in fully transitioning to single-dose syringes and do not expect any impacts to our supply or shipment timing this season."
The committee's process could be a sign of things to come
Fiscus, from the Association of Immunization Managers, says the committee's decision to only recommend single-dose flu shots without thimerosal shows that it is willing to make a decision without following protocol and considering the scientific evidence.
"Is this now going to be the standard?" she says. "That's very concerning if that's where this is heading."
Goodman also worries that these moves will ultimately undermine confidence in vaccines in the United States and abroad.
"One of my real concerns is that although the flu vaccines aren't perfect, they save lives. When they're given, they're still underutilized," he says. "If there's more difficulty in accessing them or affording them, at the end of the day, people would get hurt."
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