A few years ago a book said vaccines contained mercury and this caused many children to be autistic. But a recent editorial said that mercury is no longer used in vaccines, and besides it was safe all along anyway. But if it was safe all along, why’d they take it out? Why’d they put it in in the first place? —Darren Provine, via e-mail

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  While expounding the other day on the lost antiseptic mercurochrome, I mentioned that vaccines once contained an antibacterial and antifungal agent called thimerosal. Keeping vaccines safe is a good thing—nobody wants a repeat of the 1928 Australian case where a dozen kids died from staph-infected diphtheria vaccine drawn from a multidose vial. Unfortunately thimerosal, like mercurochrome, has the drawback of containing mercury, a toxin known to cause neurological disorders. Children are especially vulnerable. In 1999 the American Academy of Pediatrics and the U.S. Public Health Service determined that standard childhood vaccinations could lead to a dangerous accumulation of mercury. They called for thimerosal’s elimination from vaccines, and within a few years it was mostly gone.



 Controversy over thimerosal persists, though. One concern is a suspected link, often circulated on parenting and fringe-science Web sites, between vaccines containing the compound and autism in children. Autism diagnosis has become more common lately, and nobody knows why—could just be improved diagnostic methods and such. Most major studies at any rate have failed to show a connection between autism and thimerosal. A 2000 vaccine safety study initially showed an association, but after errors in the analysis were corrected the link vanished. In 2002 a Danish study of more than 500,000 children over age seven found no link between measles, mumps, and rubella (MMR) vaccinations and autism. A 2003 follow-up study focusing on thimerosal-containing vaccines again showed no link, and also showed autism rates increasing despite removal of thimerosal from vaccines. A British study of 5,763 children in 2004 showed only a statistically insignificant link between MMR vaccine and autism; a study of 27,749 children in Montreal found no MMR link to autism or related problems. The World Health Organization reviewed four major studies and found no link between thimerosal from immunizations and neurological disorders in children.

Update 9/11/2018: A new headline was added.