The injected flu vaccine stimulates your body's immune system to make antibodies to attack the flu virus.
Antibodies are proteins that recognise and fight off germs, such as viruses, that have invaded your blood.
If you're exposed to the flu virus after you've had the flu vaccine, your immune system will recognise the virus and immediately produce antibodies to fight it.
It may take 10 to 14 days for your immunity to build up fully after you have had the flu shot.
You need to have a flu jab every year, as the antibodies that protect you from flu decline over time, and flu strains can also change from year to year.
Read more about how vaccines work.
How the annual flu jab changes
In February each year, the World Health Organization (WHO) assesses the strains of flu virus that are most likely to be circulating in the northern hemisphere over the following winter.
Based on this assessment, WHO recommends which flu strains the vaccines should contain for the forthcoming winter. Vaccine manufacturers then produce flu vaccines based on WHO's recommendations. These flu jabs are used in all the countries in the northern hemisphere, not just the UK.
Production of the vaccine starts in March each year after WHO's announcement. The vaccine is usually available in the UK from September.
Read about the 2017/18 injected flu vaccine ingredients.
Types of flu virus
There are three types of flu viruses. They are:
- Type A flu virus – this is usually the more serious type. The virus is most likely to mutate into a new version that people are not resistant to. The H1N1 (swine flu) strain is a type A virus, and flu pandemics in the past were type A viruses.
- Type B flu virus – this generally causes a less severe illness and is responsible for smaller outbreaks. It mainly affects young children.
- Type C flu virus – this usually causes a mild illness similar to the common cold.
Most years, one or two strains of type A flu circulate as well as type B.
Deactivated viruses
Flu vaccines can protect against three or four types of flu virus (usually two A types and one or two B types).
For most flu vaccines, the strains of the viruses are grown in hens' eggs. The viruses are then killed (deactivated) and purified before being made into the vaccine.
Because the injected flu vaccine is a killed vaccine, it cannot cause flu.
Read 10 myths about flu and the flu vaccine.
Flu jab ingredients
As there are lots of different flu vaccines produced each year, for more detailed information on ingredients ask your doctor or nurse for the patient information leaflet for the specific vaccine being offered.
Read more about vaccine ingredients.
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