A mercury-based preservative used in some vaccines. Removed from childhood vaccines in 2001 but still used in some multi-dose flu vaccines.
Thimerosal is an organomercury compound (containing 49.6% ethylmercury) used as a preservative in multi-dose vaccine vials to prevent bacterial and fungal contamination. It has been used in vaccines since the 1930s.
In 1999, as a precautionary measure, the U.S. Public Health Service and American Academy of Pediatrics recommended removing thimerosal from childhood vaccines. By 2001, all routine childhood vaccines were thimerosal-free (except certain flu vaccines in multi-dose vials).