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.
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, only two types of adjuvants are approved for use in the United States: aluminum salts and monophoshoryl lipid A.
Squalene is an adjuvant approved in Europe, but not in the United States. Because it 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.
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.
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, has been the focus of intense scrutiny by the U.S. Congress and the news media. Thimerosal is no longer used as a preservative in any childhood vaccine with the exception of the influenza vaccine. Attention by the news media has caused some parents to fear that thimerosal contained in vaccines might have harmed their children.
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 antibiotics, DNA, egg proteins, fetal tissues,formaldehyde, human 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.