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A Project of The Annenberg Public Policy Center

What ingredients are in vaccines?

This article is available in both English and Español

All vaccines contain an active ingredient, or antigen, and tiny amounts of other substances that allow them to work, stay free of contamination and remain effective for longer. 

The antigen is a substance that prompts the body to mount an immune response, including protective antibodies. Antigens can be viruses, bacteria, parts of those or even the genetic code to produce parts of them. All viruses, bacteria or toxins used in vaccines are either inactivated or weakened so that they can’t make people sick. Manufacturers list the components of each vaccine on publicly available package inserts and anyone can check for possible allergens. 

Some vaccines may also contain adjuvants, preservatives, stabilizers and residual byproducts from the manufacturing process. All of these components are safe in the concentrations used in vaccines. Many of them are common and can be found in our bodies, drinking water, food or other products we ingest or use regularly. 

Adjuvants are substances that help vaccines work better by boosting the body’s immune response. Tiny amounts of aluminum salts, for example, have been safely used for this purpose for decades.

Preservatives are typically added to multidose vaccine vials to prevent contamination once a vial is opened. Although anti-vaccine groups falsely claim the tiny amounts of thimerosal, a preservative that contains ethylmercury, in vaccines is dangerous, it has been shown not to be harmful. Ethylmercury is not the same as methylmercury, a kind of mercury found in some fish and animals that can be toxic at high levels. Ethylmercury is safer because it’s broken down and excreted faster from the body. There is no link between thimerosal in vaccines and autism. Since 2001, thimerosal has not been used in any childhood vaccines in the U.S. other than multidose flu shots.

Stabilizers are used to keep the antigen stable during shipping and storage. These can include sugars, amino acids, proteins and gelatin.

Vaccines may also contain surfactants or emulsifiers used to keep all the ingredients blended together and prevent clumping. Anti-vaccine groups sometimes focus on an emulsifier called polysorbate 80 to spread fear. But the compound, which is safely used in vaccines, is found in much higher concentrations in many foods, including ice cream.

Trace amounts of other substances from the manufacturing process sometimes remain in vaccines. For example, many vaccines are made by growing viruses or producing proteins in chicken eggs or cell lines, so vaccines may contain DNA or proteins from those cells or other products from the growth media. Similarly, inactivated vaccines may contain formaldehyde, which is used to kill viruses. Again, the amounts of these substances in vaccines are minuscule and harmless, since the antigens are purified before being put into vials. No vaccine contains fetal cells or tissue.

No vaccines contain antifreeze, despite claims to the contrary. The confusion often stems from the fact that antifreeze contains ethylene glycol, and some vaccines contain polyethylene glycol. While the names are similar, the compounds are very different. The one in vaccines is nontoxic and found in a variety of skin products and medicines.