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 – 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.
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.
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
Thiomersal is no longer used in any of the vaccines routinely given to babies and young children in the NHS childhood immunisation programme.
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.
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.
Most killed vaccines contain a very small amount of aluminium-based adjuvant.
Although aluminium can be toxic in large quantities, no harmful effects are seen with the level of aluminium used in such small amounts in vaccines.
Squalene oil is the adjuvant added to the trivalent flu vaccine for adults aged 65 and over.
It comes from fish oil and is highly purified before being used in the flu vaccine.
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.
There have been a small number of allergic reactions to vaccines containing gelatine.
People with a known allergy to gelatine are advised to consult their doctor before receiving a vaccine that contains it.
Some religious groups, such as Muslims and Jews, may be concerned about using vaccines containing gelatine from pigs.
But many faith group leaders have stated the use of gelatine in vaccines is acceptable and does not break any religious rules.
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.
The albumin is produced by cells, such as yeast cells, that have had the gene for human albumin inserted into them.
The cells are then able to generate large quantities of human serum albumin without any need to extract it from human blood.
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
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.
The live nasal spray flu vaccine given to children has a very low egg protein content. It can be safely given to children with an egg allergy.
Children and adults who have previously had a very severe allergic reaction to eggs may be advised to have their flu vaccine in a hospital.
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.
Children and adults with a severe egg allergy can safely receive the MMR vaccine.
Formaldehyde in vaccines
Formaldehyde is a chemical also used in the production of killed vaccines.
It's used very early in the manufacturing process to kill or inactivate the toxins from bacteria or viruses.
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 would be exposed to in vaccines.
Antibiotics in vaccines
Antibiotics are added to some vaccines to prevent the growth of bacteria during the production and storage of the vaccine.
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.
But tiny amounts of an antibiotic called neomycin, which is capable of triggering an allergic reaction, are found in:
- MMR vaccine
- 6-in-1 vaccine
- quadrivalent inactivated flu vaccine
- 4-in-1 pre-school booster vaccine Repevax
- shingles vaccine
Anyone known to be allergic to neomycin or any other antibiotic should consult their doctor before having a vaccine.
Page last reviewed: 13/03/2016
Next review due: 13/03/2019