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,
  vaccine
Asked by grozmanas to Stephen, Lauren, Joseph, Eoin, Colm on 16 Nov 2013.
- Joseph Roche answered on 16 Nov 2013:
  
  The vaccines are manufactured each year to take into account the three most common flu viruses. Its not an exact science, but it works pretty well.

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