How the flu vaccine works

When a flu vaccine is given it stimulates your body's immune system to make antibodies to attack the flu viruses that are contained in the vaccine.

Antibodies are proteins that recognise and fight off germs, such as viruses, that have invaded your blood. It can take 10 to 14 days for immunity to build up fully after having a flu vaccine, so it is important to have a flu vaccine before flu starts to circulate.

No vaccine is 100% effective, but 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 is likely to recognise the virus and produce antibodies to fight it.

If you are in a risk group you should have a flu vaccine every year, as the antibodies that protect you from flu decline over time, and the flu strains circulating in the community 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 the following winter.

Based on this assessment, WHO recommends which flu strains the vaccines for the forthcoming winter should contain. Vaccine manufacturers then produce the flu vaccines. Production of the vaccine generally starts in March each year.

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.

Flu vaccines contain three or four different strains of flu virus.

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


Click here to see all vaccination leaflets.

Last Updated: 11/11/2019 13:22:01
The information on this page has been adapted by NHS Wales from original content supplied by NHS UK NHS website