In addition to one or more antigens, there are other components in a vaccine. These include:
- adjuvants: these improve the immune response to the vaccine by making the response stronger, faster and more sustained in time. An example is aluminium;
- excipients: these are inactive ingredients, like water, or sodium chloride (salt), as well as the preservatives or stabilisers that help the vaccine remain unchanged during storage, keeping it active.
These substances are consistently controlled to ensure they are present at levels that have been shown to be safe. Regulators check that their benefits are weighed against the risk of any reactions to them.
There may also be trace amounts of other substances used in the manufacturing process, such as ovalbumin (a protein found in eggs) or neomycin (an antibiotic).
Whenever these substances are present at a level that might trigger a reaction in a sensitive or allergic individual, their presence is declared in the information provided to healthcare workers and patients on the vaccine. For example, the package leaflet will state if there are special precautions for use of a vaccine in people with certain allergies, such as vaccines including trace amounts of egg in people with an egg allergy.