In addition to the antigen, a vaccine may contain a range of other substances (eg, an adjuvant and a preservative). Traces of residual components from the manufacturing process may also be present in the vaccine.
Adjuvants are substances that enhance the immune response to an antigen by a range of mechanisms, including improving the delivery of the antigen to the innate immune system and to the lymphoid organs. Use of adjuvants also means that less antigen (which can be difficult to produce) is needed (antigen sparing).
Previously the only adjuvants licensed for human use were aluminium salts such as aluminium hydroxide and aluminium phosphate. Other adjuvants now in use include oil-in-water emulsions (MF59, Novartis; AS03, GSK), a bacterial endotoxin (AS04, GSK) and one that uses immunopotentiating reconstituted influenza virosomes. Most non-live vaccines require an adjuvant, and most vaccines still use aluminium adjuvants.
See chapter 3 for further information on vaccine content.
Preservatives prevent the contamination of vaccines, particularly in multi-dose vials. 2-phenoxyethanol is an example of a preservative used in some vaccines. It is also used in many cosmetics and baby care products. Many vaccines do not contain a preservative. Mercury-based preservatives (thiomersal) are no longer used in vaccines on the New Zealand National Immunisation Schedule.
Stabilisers protect the vaccine from adverse conditions (such as exposure to heat), inhibit chemical reactions and prevent components from separating. Examples include sucrose, lactose, albumin, gelatin, glycine and monosodium glutamate (MSG).
These are wetting agents that alter the surface tension of a liquid, like a detergent does. Surfactants assist particles to remain suspended in liquid, preventing settling and clumping. A commonly used surfactant is polysorbate 80, made from sorbitol (sugar alcohol) and oleic acid (an omega fatty acid). It is also commonly used in foods such as ice-cream.
Residuals are traces of substances that remain in the vaccine as an inevitable consequence of the manufacturing process, and because the concentrations are so low there is no reason to remove them. Regulatory bodies vary as to which trace substances must be specified. Some manufacturers choose to list all of them. Residuals may include virus-inactivating agents (such as formaldehyde), antibiotics and other substances used in the manufacturing process, such as egg protein and gelatin.