Other Vaccine Ingredients

Vaccines may contain substances to maintain potency at less-than-optimal temperatures or to preserve sterility during production or after they are opened.

In addition to antigens and adjuvants, common vaccine components include the following:

Suspending Fluid

Active vaccine components typically are suspended in sterile water, saline, or protein-containing liquids. Some vaccines are stored as freeze-dried powders and must be mixed with a liquid called a diluent before they can be administered. Diluents are specifically designed for each vaccine to ensure proper safety and potency.

scientist holding a vial of freeze-dried vaccine
A scientist holds a vial of freeze-dried vaccine.


Stabilizers are additives that protect vaccines from adverse conditions like high temperatures. Commonly used stabilizers include the sugars sucrose and lactose, the amino acid glycine, and monosodium glutamate, an amino acid salt. Proteins such as human or bovine serum albumin and gelatin also are used as stabilizers.


Preservatives such as phenol, phenoxyethanol, and thimerosal may be included in vaccines to prevent bacterial or fungal contamination. Phenol is a disinfectant and antiseptic. Phenoxyethanol is chemically related to phenol and also is used as a preservative in many cosmetics. Thimerosal is a mercury-containing compound that has been used as a preservative in some vaccines since the 1930s.

One product of the metabolism, or degradation within the body, of thimerosal is ethyl mercury, an organic derivative of mercury. A related compound found in many fish and shellfish, called methyl mercury, can be toxic to people at high levels (see a mercury exposure comparison chart). Due to concerns about mercury exposure, thimerosal was removed from most childhood vaccines by 2001 as a precautionary measure. Subsequent studies have shown that, unlike methyl mercury, ethyl mercury is quickly eliminated from the body and does not build up to toxic levels.

Learn more about thimerosal in vaccines .

Trace Components

Some vaccines may contain residual quantities of components used during the manufacturing process, including inactivating agents, antibiotics, and cellular residuals. These agents are removed at the end of the manufacturing process, but trace amounts may be present in some vaccines.

Inactivating agents are used to eliminate the harmful effects of bacterial toxins or to render viruses incapable of causing disease. Examples of inactivating agents include formaldehyde and β-propiolactone. Formaldehyde is a naturally occurring chemical found in plants, animals, and the human body, where it plays a role in the synthesis of DNA and amino acids (the building blocks of proteins). Although prolonged exposure to high levels of formaldehyde can be harmful, the amount of the chemical present in certain vaccines is much lower than the amount naturally present in the body. Exposure to β-propiolactone also has been linked to health concerns. However, the chemical breaks down quickly in water-based solutions, and the very small amount of residual β-propiolactone in some vaccines is completely broken down and harmless.

scientist checking chick embryo development before inoculating eggs with influenza virus suspension
A scientist checks chick embryo development before inoculating eggs with influenza virus suspension.

Antibiotics sometimes are added to vaccines to ensure that bacterial contamination does not occur during the manufacturing process. Antibiotics used for this purpose include neomycin, streptomycin, polymyxin B, chlortetracyline, and amphotericin B. The antibiotics that are most likely to cause allergic reactions—penicillins, cephalosporins, and sulfonamides—are not used in vaccine manufacture.

Vaccine viruses can be produced in cells or cell culture, and trace amounts of cellular materials may be present in the final vaccine. The viruses incorporated into influenza and yellow fever vaccines, for example, are grown in chicken eggs, and the vaccines may contain traces of egg proteins. In general, people with egg allergy can receive these vaccines safely. In January 2013, the Food and Drug Administration approved the first egg-free flu vaccine, called Flublok. The antigens included in Flublok are produced in laboratory-grown insect cells.

Measles and mumps vaccine viruses are grown in chicken embryo tissue cultures. Studies have shown that the measles, mumps, and rubella (MMR) and measles, mumps, rubella, and varicella (MMRV) vaccines can be given safely to egg-allergic children, including those with severe allergy.

Other vaccine antigens, such as those used in hepatitis B vaccines and the human papillomavirus (HPV) vaccine Gardasil, are manufactured in yeast cells. These vaccines may contain small amounts of yeast proteins, which have not been shown to cause allergic reactions in people.

Content last reviewed on June 4, 2014