The main 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 germ. 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 that have been destroyed with chemicals or heat.
This process destroys the viruses' ability to replicate 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 flu jab, whooping cough vaccine, and polio vaccine.
Live vaccines – also called attenuated vaccines – contain viruses that have been weakened, but not destroyed, in a laboratory.
The viruses within live vaccines cannot cause disease in healthy people, but can still replicate to produce a strong immune response.
They cannot be given to people who are immunosuppressed, however, as there is 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 and often gives lifelong protection.
Examples of live vaccines are the BCG vaccine, shingles 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 (PIL).
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 PIL of a vaccine using its brand name.
Thiomersal (mercury) in vaccines
Thiomersal is a preservative that contains small amounts of mercury. It's used to prevent the growth of bacteria or fungi in the vaccine.
High doses of mercury can be toxic to the brain and other organs. However, no harmful effects have been linked with the level of thiomersal used in the small amounts present in vaccines.
Although there have been concerns in the past that vaccines that contain thiomersal can cause autism, there is no scientific evidence that this is the case.
The World Health Organization (WHO) has stated there is no risk from thiomersal in vaccines – read the WHO statement in full.
Thiomersal is no longer used in any of the vaccines routinely given to babies and young children in the NHS childhood immunisation programme.
Adjuvants in vaccines
Adjuvants work to boost our immune response to a vaccine and make it 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 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.
Pork gelatin in vaccines
Gelatin 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 gelatin in the UK routine vaccination schedule are the MMR vaccine, the shingles vaccine, and the children's nasal flu vaccine.
There have been a small number of allergic reactions to vaccines containing gelatin, so people with a known allergy to gelatin are advised to consult their doctor before receiving a vaccine that contains it.
Religious groups such as Muslims and Jews may be concerned about using vaccines containing gelatin from pigs. However, many faith group leaders have stated the use of gelatin in vaccines is acceptable and doesn't break any religious rules.
Read this NHS leaflet about vaccines and porcine gelatin.
Read more about religious opinion on pork gelatin in vaccines.
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 doesn't 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.
Recombinant human serum albumin is used as a stabiliser in an MMR vaccine used in the UK called MMRVaxPro.
Eggs in vaccines
Two vaccines in the UK routine schedule contain small amounts of egg protein – the MMR vaccine and the flu vaccine.
Flu vaccine is grown on hens' eggs and is capable of triggering an allergic reaction. Children and adults with an egg allergy are therefore advised to have an alternative, such as an egg-free inactivated flu vaccine.
MMR vaccine is grown on cells from chick embryos, which isn't the same as hens' eggs and therefore doesn't trigger an allergic reaction. Children and adults with a severe egg allergy can safely receive the MMR vaccine.
Formaldehyde in vaccines
Formaldehyde, widely known as an embalming fluid, 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 bacteria, virus or toxin.
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. However, 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 aren't used in vaccines.
However, the MMR vaccine contains tiny amounts of an antibiotic called neomycin, which is capable of triggering an allergic reaction. Anyone known to be allergic to neomycin should consult their doctor before receiving the MMR vaccine.