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 three 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.

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

The injectable flu vaccine contains three different types of flu virus (usually two A types and one B type).

For most flu vaccines, the three 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.

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.