Thimerosal in Vaccines
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