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What is in the vaccines?

Key facts

  • The most important part of a vaccine is the antigen which trains the immune system to recognise and quickly get rid of disease-causing bacteria or viruses.

  • The antigens in the vaccines recommended for pregnant women are ‘inactivated’ which means they cannot reproduce or cause disease.

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

Last updated on 24 April 2023.

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 recognise disease-causing bacteria or viruses and remove them from the body quickly, before they can cause serious illness.

Most antigens are fragments of the bacteria or virus that causes the disease the vaccine is targeting. The antigens in the vaccines recommended for pregnant women are ‘inactivated’ meaning that the bacteria or viruses they are made from have been killed before being introduced to the vaccine. Inactivated antigens cannot reproduce themselves or cause disease.  ‘Live’ vaccines contain ‘live’ antigens and should not be given to women during pregnancy.

What are adjuvants?

Adjuvants help strengthen the immune system’s response to the antigens in vaccines. 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 smaller than the amount found naturally in other things children consume such as breastmilk and formula1.

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.

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.

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 used. Vaccines are mostly made up of water or saline.

The development of this content was led by the Murdoch Children's Research Institute for the MumBubVax website (now archived).

  1. Children’s Hospital of Philadelphia. Aluminum in vaccines: What you should know. 2023.