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,
  vaccine,
  virus
Asked by theojoseph2 to Ed, Hayley, Jason, Nathan, Sophie on 12 Mar 2013. This question was also asked by thing2oxo.
- Jason King answered on 12 Mar 2013:
  
  Great question. This is a big problem. Viruses change really quickly so it is always a race to try and keep up. The way most vaccines work are by giving you a small dose of a weakened, or harmless version of a particular virus. This sort of teaches your body what it looks like, so when you get the proper one, that looks similar, the body is already looking out for it. It can then respond and attack it much faster. Even if the virus changes a bit, it will hopefully still look similar to your body, so it will be recognised. The problem really comes when it changes too much, and then the vaccine won’t work. The more common a virus is, the more chance it has to change, so we need to keep on isolating it, and making a weaker version again to use as a vaccine. Its a continuous process of chasing after the new version of the virus.

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