Mercury in flu vaccines. Credit: Inside Creative House, Shutterstock
A key US health panel has voted to phase out Thimerosal, a mercury-based preservative used in some seasonal flu vaccines.
The vote came on June 27, 2025, during a meeting of the Advisory Committee on Immunization Practices (ACIP), a group that guides national vaccine recommendations for the Centers for Disease Control and Prevention (CDC). The committee recommended that all children and adults should only receive single-dose flu vaccines that do not contain thimerosal.
What is Thimerosal and why is it controversial?
Thimerosal (also spelled thiomersal) is an ethylmercury-based preservative used since the 1930s to prevent bacterial and fungal growth in multi-dose vaccines. According to the CDC, it has a strong safety record.
While most childhood vaccines in the US have been thimerosal-free since 2001, the ingredient remains in some multi-dose flu vaccines, which make up about 5 per cent of total doses.
Despite its decades of use and repeated studies showing no harmful effects, thimerosal has long been a target of anti-vaccine advocates, many of whom link it to neurological harm and autism.
The ACIP’s latest vote was five in favour of eliminating thimerosal, one against, and one abstention. So why is there a desire to eradicate it?
The panel was recently reshaped by US Health Secretary Robert F Kennedy Jr, a long-time vaccine critic. Kennedy fired all 17 former members in early June and replaced them with eight new members.
Critics claim the panel’s agenda was influenced by anti-vaccine speech. A scheduled CDC presentation reviewing the scientific evidence supporting thimerosal’s safety was supposedly scrapped, and instead, a presentation was delivered by Lyn Redwood, a nurse and founder of the World Mercury Project, a precursor to Kennedy’s Children’s Health Defense. Her presentation reportedly included at least one unverified study.
Responding to concerns, panel chair Dr Martin Kulldorff – a former Harvard professor sacked for refusing the Covid-19 vaccine – said, “It’s inappropriate to dismiss a presentation just because the person does not have a PhD or MD” (Cited by The Guardian).
Dr Cody Meissner, the lone ‘no’ vote and professor of paediatrics at Dartmouth’s Geisel School of Medicine, warned that eliminating thimerosal could limit access to flu vaccines globally.
“The risk from influenza is so much greater than the nonexistent – as far as we know – risk from thimerosal,” Meissner said. “I would hate for a person not to receive the influenza vaccine because the only available preparation contains thimerosal.” (The Guardian.)
Global implications of phasing out thimerosal in flu vaccines
Although the ACIP’s recommendations only directly affect the United States, they are often adopted globally, particularly in lower-income countries where multi-dose vials are critical due to cost and storage constraints.
Thimerosal allows manufacturers to package flu vaccines in vials that serve multiple patients, cutting costs and logistical burdens. Without it, countries might be forced to switch to single-dose formulations, which are more expensive and harder to distribute.
Dr Jacob Glanville, CEO of Centivax, a San Francisco biotech company, noted, “We support any effort to remove it entirely – there are other preservatives that are not based on mercury” (Cited by Fox News).
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