How the vaccine is made
The seasonal flu vaccine contains different types of flu virus, which are grown in hens' eggs.
They are then inactivated (killed) and purified before being made into the vaccine.
There are currently three types of vaccine that are as effective as each other but made in different ways:
- The first type is made by inactivating whole viruses with organic solvents or detergents ('disrupted live' vaccines).
- The second type is made by extracting and purifying components of the flu viruses ('surface antigen' vaccines).
- The third type uses virosomes, which are the empty envelopes of flu viruses that lack the genetic material of the original virus.
Because the flu virus is continually changing and different types circulate each winter, a new flu vaccine has to be produced each year.
How the flu vaccine composition is decided
The World Health Organization (WHO) decides each February which three flu viruses are likely to be the greatest threat that year.
The decision is made by analysing several thousand flu viruses in the WHO flu laboratories around the world. These laboratories assess which strain has been dominant over the previous winter and look for evidence of new strains that have the potential to spread, and how well the current vaccine protects against them.
Production of the vaccine starts in March each year after the WHO announcement. It is available in the UK from September onwards.
How it protects you
About a week to 10 days after you have had the flu injection, your body starts making antibodies to the virus in the vaccine.
Antibodies are proteins that recognise and fight off germs that have invaded your blood, such as viruses. They help protect you against any similar viruses you then come into contact with.
The flu virus changes every year, so you need to have a flu jab annually to make sure that you are protected against the latest strain of the virus.
