Well informed

RFK junior wants to ban an ingredient in vaccines. Is he right?

Studies show that thimerosal does more good than harm

Jul 10, 2025 03:12 PM

ON JUNE 26TH a vaccine advisory panel installed by Robert F. Kennedy junior, America’s health secretary, recommended that thimerosal (also spelled thiomersal), an ingredient used in some multi-dose vaccine vials, should be removed from all flu jabs.

Mr Kennedy says that thimerosal is a neurotoxin that causes neurodevelopmental disorders, notably autism, in children. So against the chemical is he that in 2014 he published a book condemning it. His panellists seem to share his opinion: five out of seven voted in favour of the recommendation. An American ban on thimerosal now seems imminent. The evidence, however, strongly suggests this is a mistake.

Thimerosal is an antimicrobial agent that has been used for decades in multi-dose vaccine vials to reduce the risk of contamination from repeated syringe insertions. Sceptics gripe that it contains ethylmercury, a compound of mercury. Exposure to high levels of this metal has been shown to impair cognitive deve-lopment in children. When American rates of autism were found to be increasing in the 1990s, the thimerosal in childhood vaccines was closely scrutinised. As a precautionary measure, the Food and Drug Administration (FDA) recommended that thimerosal should be removed from childhood vaccines.

Since 2001 thimerosal has consequently been reduced in or removed from almost all vaccines recommended for American children aged six and under. Most now come in single-dose vials that need no preservatives. The lone holdouts were some multi-dose flu shots (about 4% of those administered in the most recent flu season) which, being cheaper and more durable than single-dose vials, are key to efficient annual mass-vaccination campaigns. In any case, FDA guidance suggests a single dose contains about as much mercury
as a tin of tuna.

The weight of scientific evidence strongly suggests that the FDA was being too cautious. A study by scientists at the University of Aarhus, published in 2003 in Pediatrics,found that rates of autism in Denmark increased despite the removal of thimerosal from all the country’s vaccines in 1992.

What’s more, large population studies in America and Europe have consistently shown no link between exposure to thimerosal and autism. Notably, a study of over 100,000 children in Britain published in Pediatrics in 2004 conclu-ded that there was no evidence for thimerosal causing neurodevelopmental disorders (tics were the only potential exception). The link to autism has been “thoroughly debunked”, says Kathryn Edwards, an expert in vaccine safety who recently retired from Vanderbilt University. In fact, the only well-established health risks are minor symptoms such as redness and swelling at the site of an injection due to an allergic reaction to the chemical.

Although thimerosal is barely used in American vaccines, a ban may still do harm. It would make the cheapest flu vaccines less available to the most poorly served communities and slow America’s response to pandemics, says Jake Scott, an infectious-disease specialist at Stanford University. Developing alternative chemicals is possible, but would take years. Dr Edwards worries that the real damage could arrive in the long term. If America’s public-health authorities begin lending credence to the unsupported beliefs of vaccine sceptics, she says, a very worrisome precedent will have been set.■