Since the 1930s, thimerosal has been added to some vaccines and other products as a preservative because it is effective in killing bacteria and in preventing bacterial contamination, particularly in multidose containers. When thimerosal is degraded or metabolized, one product is ethyl mercury, an organic derivative of mercury. The only known side effects of receiving low doses of thimerosal in vaccines have been minor reactions such as redness and swelling at the injection site.
In July 1999, U.S. Department of Health and Human Service agencies, The American Academy of Pediatrics, and vaccine manufacturers agreed that thimerosal should be reduced or eliminated in vaccines as a precautionary measure and to reduce exposure to mercury from all sources. The decision to move toward reduced or eliminated thimerosal in vaccines was based on the various Federal guidelines for methyl mercury exposure and the assumption that the health risks from methyl and ethyl mercury were the same.
Ethyl vs. Methyl Mercury
Mercury is a naturally occurring chemical element found throughout the environment. Mercury is found in three forms: as a pure metal (as found in thermometers), as inorganic salts, and as an organic derivative. Humans and wildlife are exposed to all three forms, though most of the mercury in the environment is in metallic or inorganic forms. Organic forms of mercury are more easily absorbed when ingested and some forms are eliminated from the body very slowly. Because mercury is everywhere, it is not possible to prevent all exposure to it. Exposure to high levels of mercury can be toxic.
Methyl mercury is the most common organic derivative of mercury and is mainly produced by microorganisms in water and soil. The U.S. Environmental Protection Agency’s safe exposure guidelines for mercury are based on exposure to methyl mercury.
There are important differences, however, between methyl mercury exposure and thimerosal exposure:
- Thimerosal contains ethyl mercury, which is chemically different from methyl mercury.
- The timing and route of exposure are different for these two chemicals.
People usually become exposed to methyl mercury by eating contaminated fish, and methyl mercury can accumulate in the body when contaminated fish are consumed regularly over time. Exposure to high levels of methyl mercury is toxic and can cause mental retardation, cerebral palsy, and seizures in children exposed to methyl mercury before birth. Methyl mercury is passed from the mother to the fetus before birth and to infants through breast milk. The developing brain (before birth) is the most sensitive to damage by methyl mercury.
Prior to the removal of thimerosal from childhood recommended vaccines, infants were exposed to ethyl mercury by intramuscular injection during vaccination, not by ingestion. Furthermore, infants received thimerosal from childhood vaccines that were administered days or months apart. In contrast, methyl mercury exposure, primarily from foods, tends to occur over a longer sustained period of time.
See a chart comparing mercury exposures.
Today, routinely recommended licensed pediatric vaccines currently being manufactured for the U.S. market are either thimerosal-free or contain markedly reduced amounts of thimerosal. An exception to this is the influenza vaccine, which is available in a variety of formulations, some of which contain thimerosal, while others do not. (For a list of thimerosal-containing vaccines, visit the Food and Drug Administration website.) Thimerosal remains in some vaccines given to adults and adolescents, as well as some pediatric vaccines not on the Recommended Childhood and Adolescent Immunization Schedule. Thimerosal is still a common preservative found in vaccines used outside of the United States.