What is thimerosal, and why is it in some vaccines?

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:
  • to kill the bacteria that make the vaccine itself (e.g., whole cell pertussis vaccine)
  • to kill bacteria that might enter the vaccine during the production process (e.g., influenza vaccine)
  • 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
Unless used as a preservative, thimerosal contributes little to the final concentration of thimerosal in vaccine (at most 2 to 3 micrograms of thimerosal per milliliter of vaccine), so the chief concern has centered on thimerosal as a preservative.

Although preservatives are not required for single-dose vaccine vials, preservatives are required to help prevent bacterial contamination of vaccine vials that contain many doses

Why is this? Most multi-dose vaccines come in vials that are topped with a rubber-like stopper. With vials that contain many doses of vaccine, health care workers repeatedly pass needles through the stopper when drawing up later vaccine doses into the syringe and this can let bacteria enter the vial and contaminate the vaccine.

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