The key ingredient in all vaccines is one or more active ingredients (see below). Apart from this, the main ingredient in vaccines is water. Most injected vaccines contain 0.5 millilitres of liquid, in other words a few drops. All other ingredients weigh a few milligrams (thousandths of a gram) or even less.
Unlike food products, the list of vaccine ingredients may include products used during the manufacturing process, even if they do not remain in the finished product. Added ingredients are present in very small quantities (usually a few milligrams). Products used in making vaccines or growing the active ingredients may not remain in the final vaccine at all. If they do, they are present only in trace amounts.
Vaccine ingredients can look unfamiliar. However, it is important to remember that many of the substances used in vaccines are found naturally in the body. For example, many vaccines contain salts based on sodium and potassium (see the section on 'Acidity regulators'), which are essential for life. People may think of formaldehyde as a man-made chemical, but in small quantities it is also found naturally in the bloodstream.
All vaccine ingredients are present in very small quantities, and there is no evidence that they cause harm in these amounts. The exception to this is the small number of people who may be severely allergic to a vaccine ingredient, even if it is present only in trace amounts (for example, egg proteins or antibiotics used in vaccine manufacture). If you look up some vaccine ingredients on the internet you may read that they could be harmful, but most of them are present in vaccines in amounts that are completely normal for our bodies. Even common salt (sodium chloride), which is essential for normal functioning of the body, is harmful in large quantities.
A list of vaccine ingredients can be found on the Patient Information Leaflet (PIL) and Summary of Product Characteristics (SPC) sheet for each vaccine. See the list of PILs and SPCs on our links page. For further information about vaccine ingredients, contact the manufacturer of the vaccine.