Thimerosal is a preservative that prevents bacteria from growing. Thimerosal has been widely used since the 1930s (over 75 years) in vaccines and other medications, to help prevent contamination with harmful bacteria and fungi.
The Food & Drug Administration (FDA) considers the small amount of thimerosal in some vaccines to be acceptably safe.
Thimerosal contains mercury. If you were to weigh a thimerosal molecule, about half its weight would be mercury. The mercury in thimerosal is in a form called ethylmercury. This is different from another form of mercury called methylmercury.
High levels of methylmercury in the body can harm the kidneys or nerve tissue. Humans are exposed to methylmercury when they eat certain kinds of fish or seafood.
On the other hand, the human body removes (“eliminates”) ethylmercury more rapidly than methylmercury. Because of this, ethylmercury does not build up in the body, reducing its ability to cause harm. If there were any theoretical danger from thimerosal, it would apply to young infants and pregnant women, but not to people with larger body weights (such as older children and adults).
Even though there is little evidence that thimerosal can be harmful, US vaccine manufacturers have either reduced or removed thimerosal from childhood vaccines. This decision was made to make vaccines as safe as possible.
Very few vaccines continue to contain thimerosal as a preservative. A list of thimerosal content in some US vaccines is available from The Institute for Vaccine Safety.
In developing countries, thimerosal-containing vaccines remain in wide use because thimerosal-free vaccines are often more expensive.
Multiple population-based studies show no association between immunization with thimerosal-containing vaccines and specific neurodevelopmental disorders (for example, autism, speech or language delay, attention deficit disorder).
Immunization with vaccines containing thimerosal continues to offer the full value of the vaccine without any measurable risk from mercury. The benefits of immunization far outweigh any potential risks from exposure to thimerosal-containing vaccines, in the considered opinion of both the FDA and the Centers for Disease Control & Prevention (CDC).
The Institute of Medicine's Immunization Safety Review Committee (an independent committee of expert physicians and scientists) recently released a report, Vaccines and Autism, which concluded, "the body of epidemiologic evidence favors rejection of a causal relationship between thimerosal-containing vaccines and autism."
Four large studies have compared the risk of autism in children receiving vaccines containing thimerosal to those who received vaccines without thimerosal. Each study found no association between thimerosal exposure and autism or other neurodevelopmental disorders (NDD). These studies are clear, consistent, and reproducible and show that exposure to thimerosal through childhood vaccines does not cause autism or NDD. The only evidence of harm caused by thimerosal is a small risk of allergy, skin rash, or swelling at the injection site, similar to other vaccines.