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,
  how science works,
  vaccine,
  virus
Asked by bhavinamistry to Davie, Gemma, James P, James V, Nuala on 28 Jun 2012.
- James Verdon answered on 28 Jun 2012:
  
  I have absolutely no idea. Good question. Anyone?
- James Pope answered on 28 Jun 2012:
  
  HI bgavinamistry,
  
  Every year flu is different, but it always starts in Asia, before spreading to Europe. We get a sample sent from Asia to us (and other nations) and from that we can create each years Flu Jab! It’s just luck we are ahead of where it comes from!

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