What is thimerosal, and why is it in some vaccines?
Thimerosal/Mercury Immunization Issue
Thimerosal is a compound that is 49.6% mercury by weight. Although it is not used in all vaccines (for example, it is not used in measles-mumps-rubella or chickenpox vaccines), it has been part of the manufacture of many vaccines since the 1930s. Thimerosal has been used:
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to kill the bacteria that make the vaccine itself (e.g., whole cell pertussis vaccine)
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to kill bacteria that might enter the vaccine during the production process (e.g., influenza vaccine)
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as a preservative to prevent bacterial and fungal contamination of vaccines during their clinical use. In this case, thimerosal is added at the end of the production process either to the liquid vaccine itself or — in the case of dry powder vaccines — to the liquid used to dilute the vaccine