The Truth About Mercury in Vaccines

Here's why parents don't need to worry about mercury in the flu shot and other childhood vaccines.

toddler getting MMR vaccine
Photo: Oksana Kuzmina/Shutterstock

One of the biggest reasons why parents are hesitant to vaccinate their children is fear of mercury in the shots. And that’s an understandable argument: for years, vaccines did contain thimerosal, a mercury-based preservative used to thwart the growth of bacteria and fungi in vaccine vials. But thimerosal was removed from most vaccines in 2001, and it has since been shown to have no adverse health effects. To help shed some fears about immunizations, read up on the latest information about mercury in vaccines.

Is There Mercury in Vaccines?

For the most part, vaccines are a combination of water with antigens, which are bacterial components that trigger an immune response. But they also contain (or previously contained) some ingredients that worry parents. One of those ingredients was thimerosal, a preservative that breaks down into ethylmercury.

Fears about mercury stemmed from research showing that exposure to high doses can cause learning disabilities and damage the brain (which is why mercury is often blamed by those who suggest a connection between vaccines and autism). There's also the fear that mercury can compromise the immune system and kidneys. Researchers worried that, since children received multiple thimerosal-containing vaccinations in a short timeframe, their bodies were being over exposed.

Scientists later discovered that the ethyl-based mercury used in vaccines doesn't pose the same risk as methyl-based mercury (a known toxin found in fish that can damage the central nervous system). "Studies suggest that this is because the body can excrete ethyl-mercury more easily," says Dan Salmon, M.D., associate director for policy and behavioral research at the Institute for Vaccine Safety at Johns Hopkins School of Public Health in Baltimore.

Nevertheless, ethyl-mercury can still enter the central nervous system. So in an effort to reduce exposure to all forms of mercury, the AAP recommended in July 1999 that thimerosal be removed from vaccines—just to be on the safe. Thimerosal was effectively eliminated from almost every shot by 2001 "as a precaution,” says Neal Halsey, M.D., a pediatrician and director of the Institute for Vaccine Safety at Johns Hopkins University in Baltimore.

The multi-dose flu vaccine for children contains a tiny amount of thimerosal, but it's considered safe. Parents can also opt for the single-dose vaccine instead.

No Link Between Thimerosal and Autism

One of the main concerns about thimerosal was its supposed connection to autism. But ever since the mercury was removed from vaccines, experts and researchers have consistently found no link between the two.

Take a 2008 study published in Archives of General Psychiatry, which explored cases of autism in California. If thimerosal really did cause autism, you’d expect cases to decline after it was removed from vaccines in 2001. However, the opposite result proved true: autism rates continued to rise after mercury in vaccines was eliminated. Scientists and researchers concluded, then, that other factors must cause autism, says Eric Fombonne, M.D., director of the psychiatry division at Montreal Children's Hospital and a member of the National Institutes of Health advisory board for autism research programs.

Studies in countries like Canada and Denmark proved similar results—as did nine studies from the Centers for Disease Control and Prevention (CDC) between 2003 and 2012. Two other prominent organizations, World Health Organization and the Institute of Medicine, also found no link between mercury in vaccines and autism.

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