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Vaccine ingredients - Vaccinations

The main active ingredient of any vaccine is the disease-causing virus, bacteria or toxin, but a number of other components are needed to make the final vaccine as safe and effective as possible.

Vaccines contain "killed" (inactivated) or "live" versions of the disease-causing virus, bacteria or toxin. These are known as the vaccine antigen.

Both killed and live vaccines work by stimulating the immune system so it thinks it's being attacked by the active part of the vaccine.

Your body responds by producing antibodies that stay in your system to protect you in the future.

Killed vaccines

Killed vaccines (also known as inactivated or dead vaccines) contain viruses or bacteria that have been destroyed with chemicals or heat. 

This process destroys the viruses' ability to grow in the body and cause illness, but keeps it intact enough so that when it's given in a vaccine, a person's immune system can still recognise it and make a protective antibody response.

Killed vaccines generally produce a weaker immune response, so it often takes several doses or a booster to maintain your immunity.

Examples of killed vaccines are the whooping cough vaccine and polio vaccine.

Live vaccines

Live vaccines (also called attenuated or weakened vaccines) contain viruses or bacteria that have been weakened, but not destroyed, in a laboratory.

The viruses within live vaccines cannot cause disease in healthy people, but can still produce a strong immune response.

They cannot be given to people who are immunosuppressed, however, as there's a risk their immune system would not be able to make antibodies rapidly enough and they could develop the disease the vaccine is designed to prevent.

Because a live vaccine is the closest thing to a natural infection, it typically produces a strong immune response. It also gives long-term protection.

Examples of live vaccines are the BCG vaccineshingles vaccine, MMR vaccine and the children's nasal spray flu vaccine.

How to find out what's in a vaccine

A complete list of ingredients for each vaccine is given in the patient information leaflet.

These ingredient lists include any products used in the making of a vaccine, even though most are only needed during the production process and are removed or only found in tiny amounts in the final vaccine.

Visit the electronic Medicines Compendium (eMC) to search for the patient information leaflet for a vaccine using its brand name.

Thiomersal (mercury) in vaccines


It's also no longer used in any of the vaccines routinely given to adults in the UK.

Thiomersal is a preservative that contains small amounts of mercury. It's used to prevent the growth of bacteria or fungi in the vaccine.

Although there have been concerns in the past that vaccines that contain thiomersal can cause autism, there's no scientific evidence that this is the case.

The World Health Organization (WHO) has stated there's no risk from thiomersal in vaccines.

Read the WHO statement in full

Adjuvants in vaccines

Adjuvants are substances added to some vaccines that help to boost our immune response and make the vaccine more effective and long-lasting.

Using an adjuvant makes it possible to reduce the amount of antigen used in a vaccine, and sometimes the number of doses that need to be given.

The amount of adjuvant used in a vaccine is very small and has been shown to be safe, although they can be associated with minor reactions, such as a small temporary lump or redness at the injection site.

An example of an adjuvant is squalene oil. Squalene oil is added to the trivalent flu vaccine for adults aged 65 and over.




Most killed vaccines contain a very small amount of aluminium-based adjuvant.


  • almost all foods
  • drinking water
  • breast milk
  • baby formula milk






Pork gelatine in vaccines

Gelatine derived from pigs is used as a stabilising agent in some vaccines.

Stabilisers are added to vaccines to help protect them from the effects of heat or freeze-drying, and also help maintain the shelf life of the vaccine.

The only vaccines containing gelatine in the UK routine vaccination schedule are the shingles vaccine, the children's nasal flu vaccine and 1 of the 2 types of MMR vaccine.


People with a known allergy to gelatine are advised to consult their doctor before receiving a vaccine that contains it.



Read this NHS leaflet about vaccines and porcine gelatine

Human serum albumin in vaccines

Human serum albumin is a substance from human blood. It's a protein used to stabilise a vaccine and maintain its quality during storage.

The serum used in vaccines comes from screened blood donors, and the manufacturing process ensures that any risk of transmitting disease is eliminated.

Human serum albumin is used as a stabiliser in a chickenpox vaccine called Varilix.

Vaccines can also be produced with recombinant albumin, which does not contain any human or animal products.



One of the MMR vaccines used in the UK (MMRVaxPro) may contain a very small amount of recombinant human serum albumin used as a stabiliser.

Eggs in vaccines

In the UK routine schedule there are 2 vaccines that contain small amounts of egg protein: the MMR vaccine and the flu vaccine.

Flu vaccine is grown on hens' eggs and potentially may trigger an allergic reaction in people with an egg allergy.

Children and adults with an egg allergy are therefore advised to have either an egg-free inactivated flu vaccine or a vaccine with a very low egg protein (ovalbumin) content.



MMR vaccine is grown on cells from chick embryos, which is not the same as hens' eggs and therefore does not trigger an allergic reaction.


Formaldehyde in vaccines



Once the antigens are inactivated, the formaldehyde is diluted out, but it's possible trace amounts may remain in the final vaccine.

Formaldehyde can be harmful in high concentrations. But there are no health concerns about the small amounts found in vaccines.

Formaldehyde can be found naturally in our bloodstream. It helps with metabolism, and is present at levels far higher than we'd be exposed to in vaccines.

Antibiotics in vaccines


They can only be found in tiny amounts in the final vaccine.

Antibiotics associated with allergic reactions, such as penicillin, generally are not used in vaccines.


Anyone known to be allergic to neomycin or any other antibiotic should talk to their doctor before having a vaccine.

Page last reviewed: 18 February 2019
Next review due: 18 February 2022