Inactivated influenza vaccine used in the United States may contain thimerosal, a preservative that contains a very small amount of mercury. There's no evidence that thimerosal in vaccines causes any health problems, according to the Centers for Disease Control and Prevention (CDC).

In 1999, the U.S. Public Health Service and other groups recommended reducing the thimerosal content in vaccines to decrease total mercury exposure, especially in infants and pregnant women. Today, with the exception of some flu vaccines, none of the routine vaccines given in childhood contains thimerosal.

According to the CDC, removal of thimerosal in influenza vaccine is a complicated process. The production of flu vaccine without thimerosal will be increased as vaccine manufacturing capabilities are expanded. In the meantime, it's important to keep in mind that the benefits of influenza vaccine far outweigh the theoretical risk, if any, for exposure to thimerosal. Each year, about 36,000 people in the United States die of influenza.