The federal government may soon suggest that individuals receive flu vaccinations specifically formulated without thimerosal, a controversial preservative.
Although thimerosal has been excluded from most flu vaccines for nearly two decades, it found itself on the agenda of a committee that provides guidance to the Centers for Disease Control and Prevention (CDC) regarding vaccine policies.
This meeting, held in June, marked the first convening since Health Secretary Robert F. Kennedy Jr. dismissed all previous members of the Advisory Committee on Immunization Practices (ACIP) and replaced them with a new group of seven.
Thimerosal has faced scrutiny from groups questioning the safety of vaccines, despite an extensive history of safe usage as a vaccine preservative, as stated by the Food and Drug Administration (FDA).
In a series of votes, ACIP recommended that children, pregnant women, and all adults should receive thimerosal-free single-dose flu immunizations.
The next step lies with Kennedy or the CDC, who will need to act on these recommendations.
Dr. Jesse Goodman, a former chief scientist at the FDA now at Georgetown University, expressed concern, stating, “It’s sort of like they turned this meeting into solving a problem that doesn’t really exist anymore.”
The only dissenting vote came from Dr. Cody Meissner, a professor of pediatrics at Dartmouth College.
He remarked, “Of all the issues that ACIP needs to focus on, this is not a big issue. The risk from influenza is so much greater than the non-existent, as far as we know, risk from thimerosal.”
Meissner also stated, “There is no scientific evidence that thimerosal has caused a problem.”
So, what exactly is thimerosal, and what could be the implications if it is effectively prohibited in flu vaccines? Here are four essential points to consider.
1. **It’s a Preservative that Contains Mercury**
Thimerosal is a chemical compound, approximately 50% mercury by weight, according to the FDA.
This compound has been employed in trace amounts as a preservative in vaccines and medicines since the 1930s.
Thimerosal is used in multi-dose vials of flu vaccines to prevent contamination during use and keep patients safe, as explained by Dr. Michelle Fiscus, chief clinical officer of the Association of Immunization Managers.
Fiscus elaborated, “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. And we don’t want those vaccines to be contaminated.”
Though the majority of flu vaccines are now available in single-dose prefilled syringes, multi-use vials are often more economically viable and can conserve refrigerator space.
“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 added.
Nevertheless, thimerosal is becoming increasingly rare.
According to the CDC, during the 2024-25 flu season, 94% of flu shots were administered without thimerosal or with reduced levels of it.
2. **Thimerosal’s Usage Declined After 2001**
Following the 1997 FDA Modernization Act, the government was prompted to evaluate the presence of mercury in childhood vaccines.
However, it’s essential to note that mercury can manifest in various forms.
Methylmercury, commonly found in seafood, is known for its toxicity, while ethylmercury—the kind present in thimerosal—has a different risk profile.
Fiscus explained, “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. And ethylmercury is where thimerosal is derived.”
Though thimerosal was removed from most childhood vaccines in 2001, this decision came amid concerns rather than evidence of harm to vaccine recipients.
3. **Discredited Links to Autism**
Widespread concern that vaccines could cause autism originated from a now-discredited 1998 study led by Dr. Andrew Wakefield, which specifically examined the MMR vaccine for measles, mumps, and rubella.
Following the study’s unraveling, Wakefield lost his medical license, and The BMJ, a notable medical publication, denounced it as an “elaborate fraud” in 2011.
Nonetheless, speculation regarding a connection between vaccines and autism persisted, extending to thimerosal, despite numerous studies debunking such links.
A 2004 report by the National Academies of Sciences Engineering and Medicine reaffirmed that thimerosal was not correlated with autism.
Subsequent research in 2006, 2007, 2009, and 2010 further validated these findings, demonstrating that there was no association between thimerosal and autism or neurophysiological issues, according to the CDC.
Importantly, Goodman from Georgetown University highlighted that while thimerosal’s presence diminished in vaccines, autism rates have continued to rise over the last two decades.
This pattern undermines the argument claiming a relation between childhood vaccines and autism.
4. **Minimal Impact Expected from Thimerosal Ban**
Since the majority of flu vaccines are delivered using single-dose syringes, the average person likely won’t notice if the CDC chooses to effectively eliminate thimerosal this fall and winter.
However, potential logistical challenges could arise as health centers typically place orders for flu vaccines in February for the upcoming season.
Manufacturers might need to adjust their offerings in response to the ban.
Sanofi, which produces a multidose vaccine called Fluzone alongside single-dose syringes, stated that only a small fraction 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 preferences for this season,” the company said.
Seqirus, another manufacturer of multidose flu vaccines with thimerosal, echoed these sentiments.
They noted that thimerosal-containing vaccines represent a very small portion of their overall supply and expressed commitment to facilitating a transition to single-dose syringes, anticipating no adverse effects on supply or shipping timelines for the season.
The recent developments in the ACIP’s process may signify a shift in the future of vaccine policy.
Fiscus, from the Association of Immunization Managers, expressed that the committee’s recommendation for single-dose, thimerosal-free flu shots indicates a willingness to overlook scientific evidence in favor of a decision.
“Is this now going to be the standard?” she questioned, expressing concern over potential implications.
Goodman raised similar worries, emphasizing that such decisions might ultimately harm public confidence in vaccines, both domestically and internationally.
image source from：npr
