What is thimerosal? Is it safe when used in some vaccines?

Thimerosal is a preservative. Preservatives (such as thimerosal) are required to be used when certain vaccines are prepared in vials that hold more than one dose. Thimerosal prevents accidental contamination with bacteria or fungi that might occur when syringes repeatedly puncture the vials to withdraw a dose

Studies have shown that there is no known harm from thimerosal preservative-containing vaccines. In 1999, FDA conducted a review of thimerosal in childhood vaccines and found no evidence of harm from the use of thimerosal as a vaccine preservative, other than a reaction at the injection site. The Institute of Medicine’s Immunization Safety Review Committee reached a similar conclusion in 2001, based on a review of available data, and again in 2004, after reviewing studies performed after its 2001 report. Since then, additional studies have been published confirming these findings.

Since 2001, no new vaccine licensed (approved) by FDA for use in children has contained thimerosal as a preservative, and except for some formulations of influenza vaccine, all vaccines routinely recommended by the Centers for Disease Control and Prevention for children under six years of age have been thimerosal-free, or contain only trace amounts.