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:
  disease,
  flu,
  medicine,
  vaccine
Asked by millerbei to Asif, Laura, Lena, Sean, Viv on 17 Mar 2012.
- Lena Ciric answered on 17 Mar 2012:
  
  Every year a new vaccine is developed based on the mixture of the latest and commonest flu virus strains. We’re constantly caught in a vicious circle with the viruses! 🙁
- Sean Murphy answered on 18 Mar 2012:
  
  Each year, three strains are chosen for selection in that year’s flu vaccination by the WHO Global Influenza Surveillance Network. The chosen strains are the H1N1, H3N2, and Type-B strains thought most likely to cause significant human suffering in the coming season. Due to the high mutation rate of the virus a particular vaccine formulation is effective for at most about a year. The World Health Organization coordinates the contents of the vaccine each year to contain the most likely strains of the virus to attack the next year.

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