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What's in a vaccine?

With the emergence of COVID-19, the crucial role that vaccines play in protecting our health has come into sharp focus. During this pandemic hope comes in the form of a vaccine and immunologists around the world are working to develop a vaccine that will protect us against infection by this new coronavirus. It’s important to understand and address vaccine concerns that are prominent in public discussion and may lead to hesitancy to vaccination. Here we discuss questions about the ingredients found in a vaccine.


An active ingredient provides immunity

The active ingredient of a vaccine is a harmless form of the bacteria or virus, which cannot cause disease. Vaccines contain a very small amount of an active ingredient, just a few micrograms (millionths of a gram) per vaccine. The role of the active ingredient is to introduce antigens, which are unique features of the bacteria or virus, to your immune system. This induces a specific immune response in your body without making you sick. The immune system remembers the antigen so if you encounter the real bacteria or virus in the future, your body is ready to quickly respond before you become unwell. This is called immunity.

For vaccines that are approved for use in humans, the active ingredient is either the whole bacteria or virus, which has been killed (inactivated) or greatly weakened (attenuated), or broken-down parts of the bacteria or virus (subunit). A new type of vaccine is made from genetic material (DNA or RNA) that contains information for the body to produce small amounts of the antigen protein at the site of injection. Vaccines in development against COVID-19 are exploring all types of active ingredient delivery methods, but the most common type are RNA vaccines.

Water is the main ingredient!

The most abundant ingredient in a vaccine is water. The other ingredients in a vaccine are present in very small amounts and there is no evidence that they cause harm in these quantities (with the rare exception of people with severe allergies to an ingredient). Vaccine ingredients may sound strange but many of them are naturally found in your body or in food, usually in much larger quantities than the amount used in a vaccine. 


An adjuvant boosts the immune response

Most vaccines have a very small amount of a substance added to them to help create a stronger immune response to that vaccine. These substances are called adjuvants.

Adjuvants are particularly useful for vaccines given to very young babies and older people, who have lower immune responses to vaccines. For example, squalene oil is used in the inactivated flu vaccine given to people aged over 65 in the UK and helps boost antibody production against the flu strains in the vaccine.1 Squalene oil is found naturally in plants, animals and humans – it’s made in our liver, circulates in our blood and is excreted on the surface of our skin. The flu vaccine contains less than 10mg (one thousandth of a gram) of squalene oil and no severe side effects have been associated with the vaccine, suggesting that squalene oil poses no significant risk to health.2

Aluminium salts, usually in the form of aluminium hydroxide, are used as an adjuvant in some other vaccines, for example the HPV vaccine, to strengthen and prolong the immune response the vaccine generates. 3 Aluminium is naturally found in drinking water, breast milk and nearly all food. The amount of aluminium in vaccines is extremely small, less than 2mg, and a recent study found that, in an infant’s first year of life, the total amount of aluminium in both vaccines and food is less than the weekly safe intake level.4  Aluminium salts are also found in many other medicines, such as heartburn medication, at much larger quantities than in vaccines.5

Preservatives and stabilisers keep the vaccine clean and effective

Very small amounts of preservatives and stabilisers are used in vaccines to maintain quality and ensure the vaccine is safe to be transported and stored. These ingredients are often naturally found in the body or in food at much higher levels than in a vaccine. Preservatives are added to vaccines to prevent unwanted contamination, much like they’re used in food products to stop them from spoiling. Stabilisers are also used in vaccines to stop the components separating or sticking to the vial during transportation and storage.

Sorbitol is a sweet carbohydrate and is used in some vaccines to prevent the particles settling and clumping. It’s naturally found in fruit; a peach contains almost one hundred times more sorbitol than a vaccine.

Gelatine is used in some live vaccines, such as the shingles vaccine, to prevent the vaccine components separating and so the virus in the vaccine isn’t affected by heat during transport and storage. The gelatine used in vaccines is derived from pigs but is highly purified and broken down into very small molecules so that no pig DNA remains. Members of some faiths may however be concerned about using vaccines containing pig-derived gelatine. According to Jewish laws, porcine products in non-oral products, including vaccines, cause no concern.6 Similarly, many Muslim leaders have ruled that the presence of gelatine in vaccines does not break religious dietary laws due to its high purification and non-oral administration.6

Residual traces of components used in making the vaccine

Residuals are tiny quantities of substances that have been used during vaccine manufacture. They may not remain in the final vaccine at all or are only present as traces and often measured as parts per million or billion in the final vaccine. Residuals depend on the manufacturing process of each individual vaccine, for example how the bacteria or virus was grown or what methods were used to weaken or inactivate the pathogen. The virus used in the inactivated flu vaccine is grown in fertilised chicken eggs, therefore the vaccine may contain traces of egg protein. Although the content is very low, this vaccine is not recommended for those with an egg allergy.1 Formaldehyde is used in the production of some vaccines, such as the Hepatitis B vaccine, to inactivate or kill the toxins from the bacteria or virus. Formaldehyde is naturally found in many foods and in your body, where it is produced as part of your metabolic process. The trace amount of formaldehyde in a vaccine is fifty times smaller than that found in an apple.7

Vaccine ingredients ensure high quality

Vaccine ingredients may have unfamiliar names but many of them are naturally found in your body, food and all around us. It’s crucial to explore the role of each ingredient to understand that, in the very small amounts used, they are safe and are necessary to ensure a vaccine’s high quality. By answering this common vaccine question, the BSI hopes to provide evidence-based information to help everyone make informed decisions about vaccines and their health.

For a more extensive list of ingredients in each individual vaccine, you can refer to the Patient Information Leaflet (PIL) or Summary of Product Characteristics (SPC) sheet that comes with each vaccine. Both can also be found online.

Download and share our infographic

We've created a new infographic about vaccine ingredients to help explain the details discussed in this blog. They are for everyone to use and share to aid understanding on the important topic. 

Take a look and download our vacine ingredients infographic


  1. Public Health England 2020 The national influenza immunisation programme 2020 to 2021
  2. World Health Organisation Squalene-based adjuvants in vaccines (updated Dec. 2008)
  3. University of Oxford, Vaccine Knowledge Project Vaccine ingredients (updated Aug. 2019)
  4. Mitkus et al. 2011 Updated aluminum pharmacokinetics following infant exposures through diet and vaccination. Vaccine 28 9538–43 ​ 
  5. NHS Antacids (updated: Nov. 2019)
  6. Public Health England 2015 Vaccines and porcine gelatine
  7. American Council on Science and Health Apple pie, mashed potatoes and natural formaldehyde (updated Nov. 2015)


Erika Aquino, BSI Public Engagement Manager

Dr Beth Holder, Imperial College London and IMPRINT network