Vaccine Ingredients

Vaccines contain ingredients, called antigens, which cause the body to develop immunity. Vaccines also contain very small amounts of other ingredients--all of which play necessary roles either in making the vaccine, or in ensuring that the vaccine is safe and effective. These types of ingredients are listed below.

Type of Ingredient	Examples	Purpose
Preservatives	Thimerosal (only in multi-dose vials of flu vaccine)	To prevent contamination
Adjuvants	Aluminum salts	To help stimulate the body’s response to the antigens
Stabilizers	Sugars, gelatin	To keep the vaccine potent during transportation and storage
Residual cell culture materials	Egg protein	To grow enough of the virus or bacteria to make the vaccine
Residual inactivating ingredients	Formaldehyde	To kill viruses or inactivate toxins during the manufacturing process
Residual antibiotics	Penicillin, sulfa drugs	To prevent contamination by bacteria during the vaccine manufacturing process

Today, except for some flu vaccines, none of the childhood vaccines used routinely in the United States contain mercury (thimerosal) as a preservative. Although no evidence suggests that there are safety concerns with thimerosal, vaccine manufacturers stopped using it as a precautionary measure. Now it is contained in very tiny amounts only in multi-dose vials of flu vaccine. Thimerosal is necessary in vaccines that come in multi-dose vials because they require that each individual vaccine dose be drawn from the vial with a new needle and syringe. With each needle inserted, there is the potential for introducing microbes into the vial.

There is no evidence of harm caused by the small amounts of thimerosal in flu vaccines, except for minor reactions like redness and swelling at the injection site. Flu vaccines that do not contain thimerosal are available.

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