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How the flu vaccine works

The 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 have 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 vaccine.

You need to have a flu vaccination every year as the antibodies that protect you from flu decline over time, and flu strains can also change from year to year.

Find out more about how vaccines work on the Oxford University Vaccine Knowledge Project website.

Types of flu virus

There are 3 types of flu viruses:

  • 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, 1 or 2 strains of type A flu circulate, as well as type B.

How the annual flu vaccine 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.

Flu vaccines protect against 3 or 4 types of flu virus (usually 2 A types and 1 or 2 B types). Vaccine manufacturers produce flu vaccines based on WHO's recommendations.

For most flu vaccines, the strains of the viruses are grown either in hens' eggs or mammalian cells. These flu vaccines 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.

The flu vaccine cannot give you flu

The injected flu vaccine given to adults contains inactivated flu viruses, so it cannot give you flu.

The children's nasal spray flu vaccine contains live but weakened flu viruses that will not give your child flu.

Flu vaccine ingredients

As there are lots of different flu vaccines produced each year, for more detailed information on ingredients ask the GP or nurse for the patient information leaflet for the specific vaccine being offered.

A full list of any vaccine's ingredients is available on the electronic medicines compendium (emc) website.

Page last reviewed: 16 July 2019
Next review due: 16 July 2022