What is in the vaccines?

Key facts

  • All ingredients in vaccines are tested for safety. The Therapeutic Goods Administration (TGA) reviews the safety and quality of every ingredient before a vaccine is registered for use in Australia.

  • The most important part of a vaccine is the antigen, which ‘trains’ a person’s immune system to recognise and quickly get rid of disease-causing germs (bacteria or viruses).

  • Other ingredients – including components called adjuvants, preservatives, stabilisers and diluents – protect and support the antigen.

Last updated on 13 August 2025.

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Overview

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 (called residues) can also be detected in vaccines. All ingredients in vaccines are tested for safety, and this information is reviewed by the Therapeutic Goods Administration (TGA) before a vaccine is registered for use in Australia.

What are antigens?

Antigens help the immune system to recognise disease-causing germs (bacteria or viruses) and to remove them from the body quickly, before they can cause serious illness.

Most antigens are fragments of the germs (bacteria or viruses) that causes the disease the vaccine is targeting. Some antigens in vaccines are ‘inactivated’, meaning the germs (bacteria or viruses) they are made from have been killed before being introduced to the vaccine. Inactivated antigens cannot reproduce themselves or cause disease.

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

What are adjuvants?

Adjuvants are substances that help strengthen the response of the immune system to the antigens in vaccines. In some cases, this means fewer needles are needed for an adolescent to be fully protected against a disease.

The most used adjuvants are salts called aluminium hydroxide, aluminium sulphate and potassium aluminium sulphate. They are commonly referred to as ‘alum’.1 The amount of aluminium contained in vaccines is tiny.2

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 people in Australia. Vaccines now come in more affordable packaging and new technologies that do not need a preservative like thiomersal.3 Thiomersal is still used in some other parts of the world where single-dose packages are too expensive or too expensive to transport. It is still safe and effective as a preservative.

What are stabilisers?

Stabilisers are usually sugars or oils that stop 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 very small amounts remain. In such tiny amounts, these residues are harmless – and most of them 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.4

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 the smallest useful vaccine dose can be given to people. Vaccines are mostly made up of water or saline.

Please note: In SKAI Adolescent, the phrase ‘your adolescent’ refers to all guardian relationships where health decisions for an adolescent fall under your responsibility.

Drafts of this page were reviewed by members of our Consumer Advisory Group.


  1. Verma SK, Mahajan P, Singh NK et al. New-age vaccine adjuvants, their development and future perspective. Frontiers in Immunology 2023;14:1043109. Available from https://doi.org/10.3389/fimmu.2023.1043109
  2. Children’s Hospital of Philadelphia. Aluminum in vaccines: what you should know. 2023. Available from http://media.chop.edu/data/files/pdfs/vaccine-education-center-aluminum.pdf
  3. Australian Government Department of Health, Disability and Ageing. Vaccine safety. 2023. Available from https://www.health.gov.au/topics/immunisation/about-immunisation/vaccine-safety
  4. US Food and Drug Administration. Common ingredients in US licensed vaccines. January 2024. Available from https://www.fda.gov/vaccines-blood-biologics/safety-availability-biologics/common-ingredients-us-licensed-vaccines