Thimerosal is a preservative that has been used in some vaccines since the 1930's, when it was first introduced by Eli Lilly Company. It is 49.6% mercury by weight and is metabolized or degraded into ethylmercury and thiosalicylate. At concentrations found in vaccines, it meets the requirements for a preservative as set forth by the United States Pharmacopeia; that is, it kills the specified challenge organisms and is able to prevent the growth of the challenge fungi. Prior to its introduction in the 1930's, data were available in several animal species and humans providing evidence for its safety and effectiveness as a preservative. Since then, thimerosal has a long record of safe and effective use preventing bacterial and fungal contamination of vaccines, with no ill effects established other than minor local reactions at the site of injection. As a vaccine preservative, thimerosal is used in concentrations of 0.003% to 0.01%. A vaccine containing 0.01% thimerosal as a preservative contains 50 micrograms of thimerosal per 0.5 ml dose or approximately 25 micrograms of mercury per 0.5 mL dose. The use of mercury-containing preservatives in vaccines has declined markedly since 1999 (Table 1. Thimerosal Content of Vaccines Routinely Recommended for Children 6 Years of Age and Younger - (updated 7/18/2005). FDA is continuing its efforts toward reducing or removing thimerosal from all existing vaccines. Much progress has been made to date. FDA has been actively working with manufacturers, particularly those that manufacture childhood vaccines, to reach the goal of eliminating thimerosal from vaccines, and has been collaborating with other PHS agencies to further evaluate the potential health effects of thimerosal. In this regard, all vaccines routinely recommended for children 6 years of age or younger and marketed in the U.S. contain no thimerosal or only trace amounts (1 microgram or less mercury per dose), with the exception of inactivated influenza vaccine, which was first recommended by the Advisory Committee on Immunization Practices in 2004 for routine use in children 6 to 23 months of age. | 
