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What is Thimerosal?

"Although the names may sound the same, methylmercury and ethylmercury are very different. An analogy is the difference between methyl alcohol and ethyl alcohol: Methyl alcohol is antifreeze, and ethyl alcohol is a Bud light."
- Dr. Ari Brown, pediatrician and author of popular books for parents about infant and child health and development.
Food and Drug Administration
What is Thimerosal?
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. 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. Although it was not used in all vaccines (for example, it has never been used in live virus vaccines such as MMR or chickenpox vaccines), it had been part of the manufacture of many vaccines in use in the United States and other countries until recently.
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 (to view recommendation, visit MMWR's website). Thimerosal has been removed from or reduced to trace amounts in all vaccines routinely recommended for children 6 years of age and younger, with the exception of inactivated influenza vaccine. A preservative-free version of the inactivated influenza vaccine (contains trace amounts of thimerosal) is available in limited supply at this time for use in infants, children and pregnant women. Some vaccines such as Td, which is indicated for older children (≥ 7 years of age) and adults, are also now available in formulations that are free of thimerosal or contain only trace amounts. Vaccines with trace amounts of thimerosal contain 1 microgram or less of mercury per dose. 
This page was reviewed on February 25, 2011
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