Vaccine Ingredients

by CHOP  (Children's Hospital of Philadelphia)
Vaccine Ingredients - (Image Credit: WHO)


Antigens can be considered the active ingredients in vaccines because they are the parts of the vaccine to which an immune response is generated. Most often these components are whole viruses or bacteria, parts of the viruses or bacteria, or products made by bacteria, called toxins. Learn more about how vaccines are made.


Adjuvants are substances added to vaccines that allow them to work better by enhancing the immune response to the vaccine, decreasing the quantity of vaccine needed to gain protective immunity, or lowering the number of doses required. Currently, five types of adjuvants are approved for use in the United States:

  • Aluminum salts — Salts that contain aluminum have been used in vaccines since the 1930s. Find out in which vaccines aluminum is used, how much aluminum is present, and how we know it is safe.
  • Monophosphoryl lipid A — This substance was isolated from the surface of bacteria. It is modified so that it is not harmful when added to vaccines. The only vaccine in the U.S. that currently uses this adjuvant is the shingles vaccine called Shingrix®.
  • QS21— Isolated from the bark of Quillaja saponaria trees, this molecule is soap-based. It is only found in the shingles vaccine called Shingrix.
  • MF59 — A substance composed of squalene oil emulsified in water. Squalene is found in people, animals, and plants. It is used in the U.S. in a version of the influenza vaccine called FLUAD®. Because squalene is an oil-in-water emulsion, some people wonder whether they can get vaccines if they have allergies to peanut or corn oils. However, these oils are not in vaccines.
  • CpG — Two nucleic acids, Cytosine (C) and Guanine (G), which are found in DNA, are linked to form an adjuvant that is contained in a hepatitis B vaccine (Heplisav-B®).


Stabilizers are used in vaccines to protect the integrity of the active ingredients during manufacture, storage and transport. A commonly used stabilizer that can be associated with allergic responses to vaccines is gelatin.

The HPV vaccine contains polysorbate 80 as a stabilizer. Some stories on the internet have suggested that polysorbate 80 in this vaccine causes infertility. First, it is important to know that the HPV vaccine does not cause infertility. Second, polysorbate 80 has been used for many years as an emulsifier to make ice cream smooth and to slow melting. A typical serving of ice cream (1/2 cup) may contain about 170,000 micrograms* of polysorbate 80. On the other hand, the amount of polysorbate 80 in each dose of the HPV vaccine — 50 micrograms* — is very small. Therefore, polysorbate 80 isn’t contained in vaccines at quantities that could possibly do harm.
* A microgram is one-millionth of a gram, and a gram is the weight of one-fifth of a teaspoon of water.


Preservatives are used in some vaccines to prevent bacterial or fungal contamination. The requirement for preservatives in vaccines arose from many incidents in the early 20th century of children who developed severe and occasionally fatal bacterial infections after administration of vaccines contained in multi-dose vials. For example, in 1916, four children died, 26 developed local abscesses, and 68 developed severe systemic infections after receipt of a typhoid vaccine contaminated with the bacteria Staphylococcus aureus. As a consequence of this and similar incidents, preservatives have been required for vaccines contained in multi-dose vials (with some exceptions) since the 1930s.

Thimerosal, a mercury-containing preservative used in vaccines, has been the focus of intense scrutiny. Thimerosal is no longer used as a preservative in any childhood vaccine routinely used in the U.S. with the exception of some vials of the influenza vaccine. Because of the fear by parents that thimerosal contained in vaccines might have harmed their children, numerous studies have been conducted to address this concern. These large, controlled studies have not found evidence of harm. To read about some of these studies, go to the "Thimerosal (mercury) and vaccines" section of our vaccine safety references page.

Manufacturing by-products

Because vaccines are made from viruses and bacteria, some chemicals and cell by-products used during vaccine production may remain in the final preparation in minute quantities. Some examples include antibioticsDNAegg proteinsfetal tissuesformaldehydehuman proteins, and yeast.

Concerns have also been voiced about SV40, a virus found in cells used to grow the poliovirus vaccine during the 1950s and 1960s. However, these concerns were proven to be unfounded.

CDC’s Pink Book - Vaccine Ingredients

The Centers for Disease Control and Prevention (CDC) has compiled the ingredient list for most vaccines into a table. While the table may be helpful for people concerned about particular allergies or ingredients, it is important to realize that the table does not indicate quantities in each vaccine. In most cases, the quantities are so minimal that they do not cause allergic reactions or symptoms of toxicity. Further, many of these ingredients are commonly found in other products.


Also see:

Vaccine Ingredients (by University of Oxford)

The ingredient list for most vaccines (PDF document, by the CDC) 

What's in Vaccines (CDC)


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