The composition of vaccines varies slightly depending on their development process. The key component is a tiny, harmless version of the bacteria or virus being targeted, which is unable to cause illness. This component introduces antigens to the immune system to trigger a targeted response.

Water makes up the largest portion of a vaccine. Other ingredients are included in trace amounts, and there is no evidence suggesting harm at these levels, except in rare cases of severe allergies to specific substances.

Vaccines include small quantities of preservatives and stabilisers, such as sorbitol and citric acid, to preserve quality and ensure safety during transport and storage. These are naturally present in the body or in foods at much higher concentrations than in vaccines. Preservatives prevent contamination, similar to their use in food preservation, while stabilisers prevent separation or adhesion to the vial during handling.

Certain vaccines contain minute amounts of adjuvants—substances that enhance the immune response. Aluminium hydroxide, a common adjuvant, strengthens this response. Found naturally in food and water, the amount in vaccines is minimal. Research shows that the total aluminium from vaccines and diet in an infant’s first year remains below the safe weekly intake. Adjuvants are also present in larger amounts in other medications, posing no significant health risk at vaccine levels.

Formaldehyde is used during vaccine production. This organic compound occurs naturally in living organisms and is produced by the human body during metabolism. While high levels can be toxic, the residual amounts in vaccines are fifty times lower than those in a pear.

For a detailed list of ingredients in specific vaccines, consult the Patient Information Leaflet (PIL) or Summary of Product Characteristics (SPC) provided with each vaccine, both of which are also available online.