How do scientists know how to make a flu vaccine if viruses can be different every year?

Question: How do scientists know how to make a flu vaccine if viruses can be different every year?
- Keywords:
  flu,
  health,
  vaccine,
  virus
Asked by francesca_sweet to Anna, George, Jodi, Rob, Stefan on 13 Nov 2014.
- Stefan Lines answered on 13 Nov 2014:
  
  This is a clever question. My understanding is that the World Health Organisation keep an eye on what virus types are expected to be ‘going around’ the most each year. That can help. Secondly, the flu vaccination often contains more than one strain of the virus – so multiple effectiveness! Finally, even if the virus has changed slightly, the vaccine can be enough to trigger and immune response which is sufficient for your body to wipe out even a new strain of flu.

Question: How do scientists know how to make a flu vaccine if viruses can be different every year?