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. Thimerosal contains ethyl mercury. 2  This is a different chemical compound than mercury found in the environment (methylmercury). 3  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.