Why is it difficult to make flu vaccines?
Flu is a complicated virus. There are three basic types: A, B and C.
Type A is the most dangerous; it can cause serious disease and triggers worldwide pandemics. Influenza type A is the most complex. On the surface of the virus there are two types of protein shapes that help to invade the body’s cells: H and N. There are 18 different types of the H protein shape and 11 different types of the N protein shape. Flu virus A can make up to 198 different combinations of H and N proteins.
The flu virus can also change quickly and easily. The flu virus may do this through ‘antigenic drift’ – a gradual process of genetic change that leads to even more variety for each type. Different types of flu virus can also combine their genetic material to make a new sub-type – this is called ‘antigenic shift’.
Each year’s flu vaccine is made 6 months before the coming flu season using estimations about the type of virus expected to circulate. Nine out of every ten years the vaccine matches the strains causing illness that winter.
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