In addition to one or more antigens, there could be other components in a vaccine, depending on the type of vaccine.
- stabilisers: to keep vaccine components stable;
- adjuvants: these improve the immune response to the vaccine by making the response stronger, faster and more sustained over time – an example of which is aluminium;
- excipients: these are inactive ingredients, like water, or sodium chloride (salt), as well as preservatives or stabilisers that help the vaccine remain unchanged during storage, keeping it active.
All vaccine components 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.
In some types of vaccines there could 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 about 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.