The Food and Drug Administration (FDA) has approved several formulations of the seasonal flu vaccine, including multi-dose vials and single-dose units. (See Table of Approved Influenza Vaccines for the U.S. 2012–2013 Season.) Since seasonal influenza vaccine is produced in large quantities for annual immunization campaigns, some of the vaccine is produced in multi-dose vials, and contains thimerosal to safeguard against possible contamination of the vial once it is opened.
The single-dose units are made without thimerosal as a preservative because they are intended to be opened and used only once. Additionally, the live-attenuated version of the vaccine (the nasal spray vaccine), is produced in single-dose units and does not contain thimerosal.
Is thimerosal being used in other vaccines?
Since 2001, no new vaccine licensed by FDA for use in children has contained thimerosal as a preservative, and all vaccines routinely recommended by CDC for children younger than 6 years of age have been thimerosal-free, or contain only trace amounts of thimerosal, except for multi-dose formulations of influenza vaccine. The most recent and rigorous scientific research does not support the argument that thimerosal-containing vaccines are harmful. However, CDC and FDA continually evaluate new scientific information about the safety of vaccines.
Is thimerosal in vaccines safe?
Yes. There is a large body of scientific evidence on the safety of thimerosal. Data from several studies show the low doses of thimerosal in vaccines do not cause harm, and are only associated with minor local injection site reactions like redness and swelling at the injection site.
Three U.S. health agencies (The Centers for Disease Control and Prevention (CDC), the Food and Drug Administration (FDA), and the National Institutes of Health (NIH)) have reviewed the published research on thimerosal and found it to be a safe product to use in vaccines. Three independent organizations [The National Academy of Sciences’ Institute of Medicine, Advisory Committee on Immunization Practices (ACIP), and the American Academy of Pediatrics (AAP)] reviewed the published research and also found thimerosal to be a safe product to use in vaccines. The medical community supports the use of thimerosal in influenza vaccines to protect against potential bacterial contamination of multi-dose vials.
CDC - Seasonal Influenza (Flu) - Questions & Answers - Thimerosal and 2012-2013 Seasonal Flu Vaccines