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,
  immunology,
  vaccine,
  virus
Asked by beth5000 to Sam on 20 Jun 2013.
- Sam Geen answered on 20 Jun 2013:
  
  So every year the World Health Organisation (WHO) predicts what forms of flu will be the most dangerous. How do they do this? Well, diseases travel around the world as people move about. A businessman visits China, catches the flu there and gives it to their family when they fly home to Europe or North America, say. It’s typically slower than that, though, and the strains of flu take time to spread. By recording what types of flu happen all over the world, people can guess which ones are the most likely to spread and make vaccines based on that one before the disease spreads to the rest of the world.
  
  (Actually, I did not know this before I looked it up on the internet, so thanks for asking the question! It’s awesome to learn new things about the world.)

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