A United States federal vaccine panel, newly influenced by Health Secretary Robert F. Kennedy Jr, has voted to discourage the use of influenza vaccines containing thimerosal — a mercury-based preservative. This marks a significant policy change, given that thimerosal has long been considered safe by major health organisations globally. Its use has already been reduced in most vaccines, limited now to certain multi-dose flu vials. RFK Jr has repeatedly claimed a connection between thimerosal and autism, a claim that has been thoroughly discredited by decades of scientific research.
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Thimerosal has been used in vaccines since the 1930s to prevent microbial contamination in multi-dose vials. Its active component, ethylmercury, disrupts enzyme function in bacteria and fungi. While mercury is known to be toxic, the form found in thimerosal behaves differently in the body compared to environmental mercury. Although both forms affect brain cells, ethylmercury is less fat-soluble than methylmercury, meaning it accumulates in tissues at lower levels and is cleared from the body more quickly. This crucial chemical distinction led many scientists to conclude that health risks from ethylmercury were overstated when applying standards designed for methylmercury.
Nonetheless, amid rising concerns over vaccine safety and increasing autism diagnoses, thimerosal was phased out of childhood vaccines in high-income countries. In the UK, it was removed from childhood jabs between 2003 and 2005. Research following its removal, including studies from Denmark, has shown no correlation between thimerosal and autism. The change, while not scientifically driven, reflected broader attempts to maintain public trust in vaccines and reduce hesitancy, particularly as misinformation began to circulate widely.
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Despite public fears, global health agencies such as the World Health Organization continue to endorse thimerosal’s safety and efficacy, particularly for use in settings where single-dose vials are not feasible. Multi-dose vials are crucial in responding to pandemics and outbreaks, especially in resource-limited regions. Experts stress that the benefits of vaccination, even with thimerosal-containing doses, far outweigh any theoretical risks. The debate, while reignited politically, remains settled within the scientific community.
