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What is in the vaccines?

Key facts

  • All ingredients in vaccines are tested for safety.

  • The most important part of a vaccine is the antigen. The antigen trains the immune system to clear disease-causing bacteria or viruses from the body quickly, before they can cause serious illness.

  • Other ingredients, including adjuvants, preservatives, stabilisers, and diluents, are there to protect and support the antigen. 

Last updated on 5 May 2023.
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The most important part of a vaccine is the antigen. Other ingredients include adjuvants, preservatives, stabilisers, and diluents. Some of these are added to protect and support the antigen. Tiny traces of substances used in the process of producing antigens can also be detected in vaccines (residues). All ingredients in vaccines are tested for safety.

What are antigens?

Antigens train the immune system to clear disease-causing germs (bacteria or viruses) from the body quickly, before they can cause serious illness.

Most antigens are fragments of germs. Some antigens are weakened or killed germs or substances made by germs, called toxins.

Combination vaccines, given in a single needle, contain more than one antigen, which reduces the number of needles children need to be fully protected.

What are adjuvants?

Adjuvants help strengthen the immune system’s response to the antigens in vaccines. In some cases this means fewer needles are needed for a child to be fully protected against a disease.
The most commonly used adjuvants are salts called aluminium hydroxide, aluminium sulphate and potassium aluminium sulphate. They are commonly referred to as ‘alum’. The amount of aluminium contained in vaccines is tiny.1

What are preservatives?

Preservatives protect vaccines from becoming contaminated with harmful bacteria or fungi. The most common preservative used in vaccines is a tiny amount of alcohol.

Thiomersal, which is a salt that contains a tiny amount of mercury, is no longer used as a preservative in any vaccines routinely given to children in Australia. This is because the vaccines we use come in a single dose package and therefore do not need a preservative. Thiomersal is still used in some other parts of the world where single dose packages are too expensive or too expensive to transport. However, it is safe and effective as a preservative.

What are stabilisers?

Stabilisers are usually sugars or oils that prevent vaccines from going off (spoiling) or sticking to the sides of their containers or syringes.

What are residues?

Residues are tiny amounts of substances that remain in the vaccine after the manufacturing process.

Most of these substances are removed from the final vaccine product, but small amounts remain. In such tiny amounts, these residues are harmless, and most are already present in our bodies. For example, tiny traces of formaldehyde can be detected in some vaccines.

Formaldehyde is used to inactivate viruses, so they can be safely used as antigens in vaccines. Much larger amounts of formaldehyde are produced naturally in healthy human bodies than can be detected in vaccines.2

What are diluents?

The diluent used in vaccines is usually sterile water or saline (salt water). Diluents have no effect on the body. They are included in vaccines to ensure that the smallest useful dose can be given to children. Vaccines are mostly made up of water or saline.

  1. Children’s Hospital of Philadelphia. Aluminum in vaccines: What you should know. 2023.
  2. U.S. Food and Drug Administration. Common ingredients in U.S. licensed vaccines. 2019.