What is thimerosal? Thimerosal is an organic mercury-based preservative used in some vaccines, including some influenza vaccines. Thimerosal contains ethyl mercury. This is a different chemical compound than mercury found in the environment (methylmercury).
Thimerosal has been used as a preservative since the 1930s to prevent bacterial growth in vials containing multiple doses of vaccine and other medical products. Thimerosal is also used as a preservative in other medical products such as some throat and nose sprays.
Is there thimerosal in flu vaccines? Yes. The majority of influenza vaccines distributed in the United States contain thimerosal as a preservative. However, an increasing amount is thimerosal-free. While the multiple dose vials still have thimerosal to prevent bacterial growth, the single-dose preparations do not. The amount of influenza vaccine that is thimerosal-free is growing every year..