Good question, the flu vaccine isn’t actually just for one strain of influenza but three of four strains depending on the vaccine. Each year experts from around the world examine which flu viruses were present last year and the they decide which are the most likely to spread next year. The World Health Organisation then recommend which three strains vaccines should protect people against. This works most years but sometimes fluke flu virus can spread such as swine flu. Then scientists have to quickly find what strain the virus is and then make a vaccine.