What's actually in a vaccine, and are the ingredients (aluminium, preservatives) harmful?

Last reviewed: 2026-06-08

Quick answer

A vaccine's main ingredient is the antigen, the active part that trains the immune system, plus small amounts of supporting ingredients. Aluminium adjuvants boost the immune response in tiny amounts, far below what people take in daily from food and water. Preservatives such as thiomersal are used only in some multi-dose vaccines, in minute quantities assessed as safe over decades. All of these are present at levels well below any that could cause harm.

Vaccine ingredients can sound unfamiliar, but each has a purpose and the amounts are tightly assessed.

Antigen

This is the core of the vaccine. It can be an inactivated or weakened pathogen, or a small part of one, such as a protein or an mRNA instruction. The immune system uses it to learn to recognise the pathogen.

Aluminium adjuvants

Some vaccines add a tiny amount of aluminium salt to help the immune system produce a stronger, longer-lasting response. The aluminium in a dose is far below what infants take in daily from breast milk, formula or drinking water. Aluminium has been used in vaccines for decades with a solid safety record.

Preservatives

Preservatives such as thiomersal stop multi-dose vials from being contaminated by bacteria during use. Many vaccines now come in single-dose packaging and contain no preservative. Where thiomersal is used, the amount is minute and has been assessed as safe in long-term studies.

Stabilisers and trace residues

Vaccines may also contain stabilisers such as sugars or gelatin to keep them effective during storage and transport, plus minute traces left over from manufacturing. These residues are extremely low and within safe limits.

Regulation

In Hong Kong, vaccines must be registered with the Department of Health's Drug Office and meet safety, efficacy and quality requirements before use.

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