Thimerosal
(Mercury Based Preservative)

Thimerosal (also called thiomersal) is an ethylmercury‑containing preservative that began to be widely used in multi‑dose vaccine vials in the United States in the 1930s and was largely removed from routine pediatric vaccines by 2001, with continued limited use mainly in some multi‑dose influenza vaccines. The change in use reflects both evolving regulatory standards on mercury exposure and a precautionary policy response in the late 1990s, despite the absence of evidence that thimerosal in vaccines caused harm beyond local reactions.
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Early development
Thimerosal was developed in the late 1920s as an organomercury antiseptic and was shown in early animal and human studies to be effective as a preservative with low observed toxicity at the doses used.
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By the 1930s it was incorporated into a variety of biological products, including some vaccines, to prevent bacterial and fungal contamination in multi‑dose vials.