Fact Sheet: Safety of Thimerosal in Vaccines Against 2009 H1N1 Flu
"The Centers for Disease Control and Prevention (CDC) is aware that pregnant women, parents of young children, and others may have questions about the safety of thimerosal in vaccines against 2009 H1N1 flu. Here is some information to help you in making decisions.

Thimerosal Use Prevents Vaccine Contamination
Thimerosal is a mercury-containing preservative that is added to multi-dose vials (vials containing more than one dose) of vaccine to prevent contamination and growth of potentially harmful bacteria. This may occur when a syringe needle enters a vial as a vaccine is being prepared for administration. Such contamination could cause serious local reactions, illness, or death..."