Thimerosal is a mercury-containing compound that kills or prevents the growth of microorganisms. Since the 1930's, thimerosal has been used as a preservative to prevent growth of bacteria and fungi in vaccines (FDA 2005). One of the breakdown products of thimerosal is ethylmercury, an organic form of mercury. The Food and Drug Administration (FDA) Modernization Act of 1997 called for the FDA to review and assess the risk of all mercury-containing food and drugs, including vaccines (CDC 1999). The FDA noted that due to immunization, a 6-month old infant could potentially be exposed to cumulative levels of ethylmercury exceeding Environmental Protection Agency (EPA) guidelines for safe intake of methylmercury, a related mercury compound that has been more thoroughly studied than ethylmercury (CDC 2003). It is not known if toxicity for ethylmercury is the same as that for methylmercury, but in conducting safety evaluations, the FDA assumes that the two chemicals have equal toxicity (FDA 2005). According to a joint statement issued in 1999 by the American Academy of Pediatrics (AAP), the United States Public Health Service (PHS), and the American Academy of Family Physicians, there was no evidence of harm resulting from exposure to the low levels of thimerosal used in some vaccines (CDC 1999). However, the statement called for the removal of thimerosal from vaccines routinely recommended for infants as a precautionary measure.
With the exception of inactivated influenza vaccines, none of the vaccines routinely recommended for children 6 years and younger currently contain thimerosal as a preservative (FDA 2005). Some of those vaccines still contain trace amounts of thimerosal that is added during their manufacture. However, mercury exposures resulting from use of thimerosal in vaccine manufacturing processes are 50 times lower than exposures resulting from use of thimerosal as a preservative. Based on the removal or reduction of thimerosal in vaccines, it has been estimated that the maximum ethylmercury exposure an infant could receive through vaccines has been reduced by 95%. Limited supplies of thimerosal-free or thimerosal-reduced influenza vaccines are available for use in children and pregnant women and the FDA is working with manufacturers to increase those supplies in the future.
Institute of Medicine reports
Exposure to high levels of mercury can clearly damage the nervous system but harmful effects of exposure to low levels of mercury, such as those associated with use of thimerosal as a preservative in vaccines, have not been demonstrated (IOM 2001). The National Academy of Sciences Institute of Medicine (IOM) assembled a committee of scientists to examine the health implications of thimerosal in vaccines (IOM 2004a). The first IOM report was released in 2001 (IOM 2001). There were very few studies for the committee to review. The most thorough study consisted of data collected by the Centers for Disease Control (CDC) and seven health maintenance organizations (HMOs). The committee concluded that there was insufficient evidence to determine if childhood thimerosal exposure causes neurological disorders such as autism, attention deficit hyperactivity disorder (ADHD), and speech or language delay. The committee stated that the theory that thimerosal could be associated with neurological disorders in children was biologically possible and noted that more studies were needed. The Committee further stated that the decision to remove thimerosal from vaccines was a prudent action in supporting public health goals of reducing mercury exposures in infants and children.
The second report of the IOM Committee was issued in 2004 (IOM 2004b). Autism was the focus of this report. The committee reviewed several studies that were conducted following the first review, including studies from the U.S., Denmark, Sweden, and the United Kingdom. The committee concluded that the body of evidence suggested that thimerosal-containing vaccines did not cause autism. Studies designed to determine possible biologic events in the development of autism following vaccination were also reviewed. The committee concluded there is no evidence that thimerosal in vaccines affects biological processes possibly related to the development of autism.
According to the IOM (IOM 2004b), there is no need for a review of policies for recommendations or schedules of routine childhood vaccines.
