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Vaccines packaged in multidose vials contain thimerosal, a preservative that protects vaccines against contamination. Thimerosal contains a small amount ethyl mercury. Other than minor reactions like redness and swelling at the injection site, there is no evidence of harm caused by the small amount of thimerosal in vaccines. Vaccines packaged in single-dose vials and pre-filled syringes are thimerosal-free. They are usually given to children under age 3 and pregnant women because Washington law restricts the amount of thimerosal in vaccines for pregnant women and children under age 3. Most of the flu vaccines licensed by the FDA for use in the United States do not contain thimerosal.