NHS Choices

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 a flu virus after you've had the flu vaccine, if the strain is one that was in the vaccine your immune system will recognise the virus and immediately produce antibodies to fight it.

It can take 10 to 14 days for immunity to build up fully after having a flu vaccine.

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

How the annual flu vaccine changes

In around February each year, the World Health Organization (WHO) assess the strains of flu virus that are most likely to be circulating in the northern hemisphere causing health problems 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. Production of the vaccine starts in March each year after WHO's announcement.

These flu vaccines are used in all the countries in the northern hemisphere, not just the UK.  The vaccine is usually available in the UK from late September.

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.

Deactivated viruses

In the 2017/18 season most of the injectable flu vaccines contain three different types of flu virus (usually two A types and one B type) and there are also two injectable flu vaccines that contain four different types of flu virus available and these contain an extra B type virus.

The virus strains for flu vaccines 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.

Flu vaccine 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.



Click here to see all vaccination leaflets.

Last Updated: 01/04/2017 09:00:00