Q & A

Is the Mercury preservative in vaccines dangerous?

No! Thimerosal is a mercury-containing preservative that has been used in some vaccines because it has a long record of safely and effectively preventing bacterial and fungal contamination of vaccines, especially multi-dose vials. There has recently been serious debate on the safety of thimerosal due to claims that it contributes to mercury toxicity and impaired neurological development.

The organic form of mercury, methyl mercury, is the kind most often found in the water, soil, plants, and seafood. In large amounts, methyl mercury is a neurotoxin, is more easily absorbed when ingested, and is less readily eliminated from the body than are inorganic forms of mercury.

One inorganic form of mercury is ethyl mercury; thimerosal is a derivative of ethyl mercury. Ethyl mercury from thimerosal-containing vaccines is rapidly eliminated from the body in the stool and does not appear to accumulate after routine immunizations.

No evidence of harm or ill effects of thimerosal have been established other than minor local reactions (such as redness at swelling) at the injection site. There is also no evidence of any harmful effect of thimerosal-containing vaccines on neurological or psychological outcome, including autism and attention deficit hyperactivity disorder (ADHD).

Although there are no evidenced negative effects of vaccines containing thimerosal, at present, all routinely recommended vaccines for children 6 years of age and younger are available as thimerosal-free formulations or contain only trace amounts of thimerosal (≤1 microgram of mercury per dose). For comparison, 1 microgram is roughly equal to 1 particle of dust. With each breath, we inhale approximately 2000 dust particles!

The only exception is the inactivated influenza vaccine, in which there is a limited supply of the trace-amount or thimerosal-free vaccine. Even influenza vaccines with more than trace amounts of thimerosal still only have amounts that are considered safe by the FDA.

There are two reason thimerosal is being taken out of vaccines. First, multi-dose vials are largely being replaced by single-dose vials, making the risk for contamination much lower. Second, other preservatives, that do not contain any mercury, can be used in some vaccines.

No studies have shown that thimerosal in vaccines is harmful. However, thimerosal is simply being eliminated from vaccines because it can be. For more information, please see www.cdc.gov