Content on this page was developed during the 2009-2010 H1N1 pandemic and has not been updated.
- The H1N1 virus that caused that pandemic is now a regular human flu virus and continues to circulate seasonally worldwide.
- The English language content on this website is being archived for historic and reference purposes only.
- For current, updated information on seasonal flu, including information about H1N1, see the CDC Seasonal Flu website.
Fact Sheet: Safety of Thimerosal in Vaccines Against 2009 H1N1 Flu
January 4, 2010, 4:00 PM ET
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 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.
Thimerosal is a very effective preservative that has been used since the 1930s to prevent contamination in a number of products including some multi-dose vials of vaccines.
Data from 19 studies show no convincing evidence of harm caused by the low doses of thimerosal in vaccines, except for minor local injection site reactions like redness and swelling at the injection site. For more information on thimerosal and its safety, visit: General Questions and Answers on Thimerosal
Since influenza vaccines are produced in large quantities for annual immunization campaigns, some vaccines are produced in multi-dose vials. FDA licensed (approved) both multi-dose vials and single-dose units of the 2009 H1N1 flu vaccine. The multi-dose vials of 2009 H1N1 flu vaccine contain thimerosal, while injectable single-dose units do not. In addition, the live-attenuated version of the vaccine, which is called LAIV and administered as a nasal spray, is produced in single-units and does not contain thimerosal.
Some priority groups may not be able to find injectable thimerosal-free 2009 H1N1 flu vaccine due to a recent recall of pre-filled, single-dose syringes manufactured by Sanofi Pasteur. Since LAIV is only approved for people from 2 through 49 years of age who are not pregnant and do not have certain health conditions, this means pregnant women and children from ages 6 through 23 months may have difficulty obtaining a thimerosal-free 2009 H1N1 vaccine. Remember, it is safe for children and pregnant women to receive a flu vaccine that contains thimerosal.
It is important to keep in mind that severe illness and possible death can be associated with influenza, and vaccination is the best way to prevent influenza infection and its complications. Currently, the 2009 H1N1 flu virus (sometimes called “swine flu”) seems to be causing serious health outcomes for the following priority groups:
- Healthy young people from birth through age 24
- Pregnant women
- Adults 25 to 64 who have certain underlying medical conditions
Children, especially those younger than 5 years of age and those who have high risk medical conditions are at increased risk of influenza-related complications.
CDC places a high priority on vaccine safety, surveillance, and research. CDC is aware that the presence of the preservative thimerosal in some vaccines and misconceptions of a relationship to autism has raised concerns. These concerns make the decisions surrounding vaccinations confusing and difficult for some people, especially some parents. Most research done in the United States, and around the world, shows no link between thimerosal in vaccines and autism, a neurodevelopment disorder. In fact, sadly, autism rates have actually gone up since thimerosal was taken out of childhood vaccines in 2001, providing further evidence that thimerosal-containing vaccines are not related to autism.
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