What’s in Vaccines?

Today’s vaccines use only the ingredients they need to be as safe and effective as possible.

Each ingredient in a vaccine serves a specific purpose:

  • provide immunity (protection)
  • keep the vaccine safe and long lasting
  • for the production of the vaccine
Vaccine Ingredients.
Type of Ingredient Example(s) Purpose Most common source found…
Preservatives Thimerosal (only in multi-dose vials of flu vaccine)* To prevent contamination From eating foods such as certain kinds of fish, mercury (which thimerosal contains) gets into the body
Adjuvants Aluminum salts To help boost the body’s response to the vaccine From drinking water, infant formula, or use of health products such as antacids, buffered aspirin, and antiperspirants
Stabilizers Sugars, gelatin To keep the vaccine effective after manufactured. From eating food such as Jell-O® and resides in body naturally
Residual cell culture materials Egg protein^ To grow enough of the virus or bacteria to make the vaccine From eating foods containing eggs
Residual inactivating ingredients Formaldehyde† To kill viruses or inactivate toxins during the manufacturing process Resides in body naturally (more in body than vaccines). Also found automobile exhaust, and household furnishing such as carpets and upholstery.
Residual antibiotics Neomycin To prevent contamination by bacteria during the vaccine manufacturing process Antibiotics that people are most likely to be allergic to — like penicillin — aren’t used in vaccines

* Thimerosal has a different form of mercury (ethylmercury) than the kind that causes mercury poisoning (methylmercury). It’s safe to use ethylmercury in vaccines because it’s processed differently in the body and it’s less likely to build up in the body — and because it’s used in tiny amounts. Even so, most vaccines do not have any thimerosal in them.  Learn more about thimerosal, mercury, and vaccine safety.

^ Because influenza and yellow fever vaccines are both made in eggs, egg proteins are present in the final products. However, there are two new flu vaccines now available for people with egg allergies. People who have severe egg allergies should be vaccinated in a medical setting and be supervised by a health care professional who can recognize and manage severe allergic conditions.

†Formaldehyde is diluted during the vaccine manufacturing process, but residual quantities of formaldehyde may be found in some current vaccines. The amount of formaldehyde present in some vaccines is so small compared to the concentration that occurs naturally in the body that it does not pose a safety concern.

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Most vaccines do not contain thimerosal or mercury. If you’re concerned about thimerosal or mercury in vaccines, talk with your child’s doctor.

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Page last reviewed: August 5, 2019