The World Health Organization insisted Friday that there was no evidence of any harm from an important vaccine ingredient, which a US medical panel voted to oppose this week.

“There is no evidence of harm from the use of thimerosal,” WHO vaccine chief Kate O’Brien told a press conference.

Her comment came after a US medical panel newly appointed by Health Secretary Robert F. Kennedy Jr. voted on Thursday to oppose long-held recommendations backing vaccines containing the preservative thimerosal, which the anti-vaccine movement falsely links to autism.

Thimerosal, a preservative that prevents bacterial and fungal contamination in multi-dose vials, has been extensively studied, with authorities finding no evidence of harm beyond minor injection-site reactions.

Although 96% of US flu vaccines in the 2024-2025 season did not contain thimerosal, the preservative remains important in lower income countries because they are more likely to use lower cost multi-dose vials that must be punctured repeatedly, raising the risk of contamination.

Thimerosal contains an artificial form of mercury called ethylmercury that is cleared from the body far more quickly than the form of the chemical found in nature. US manufacturers voluntarily removed it from most pediatric vaccines in 2001.

“For some of the vaccine supply, this is a very important ingredient in order to assure that we have the doses that are needed to protect children from serious and life-threatening diseases,” said O’Brien, speaking from the UN health agency’s headquarters in Geneva.

She said that “an extensive effort has made to remove thimerosal, out of an abundance of caution, from the vaccine supply”.

But it was important that “the public understands that there has been a thorough review of the evidence around the safety of thimerosal, and that we continue to need vaccines that… have thimerosal in them”.

“This evidence has been reviewed thoroughly.”

Although the substance is now rarely used in US vaccines, the recommendations by the influential Advisory Committee on Immunization Practices alarmed experts, who say the move has effectively embedded talking points championed by the anti-vaccine movement into national policy.

Kennedy—who spent decades spreading vaccine misinformation before becoming President Donald Trump’s top health official—abruptly fired all 17 ACIP members earlier this month, accusing them without evidence of conflicts of interest.

Across three votes, his new panelists recommended that thimerosal be removed from influenza vaccines for children, pregnant women and finally all adults.

© 2025 AFP