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

Question: How do scientists know how to make a vaccine if viruses can be different every year?
- Keywords:
  disease,
  medicine,
  vaccine,
  virus
Asked by to Daren, Lynne, Phillip, Simon on 22 Jun 2014. This question was also asked by .
- Daren Fearon answered on 22 Jun 2014:
  
  Some vaccines are what is known as live, attenuated versions of the virus. These are versions of the virus that have been manipulated in a lab to stop them spreading disease. Another type of vaccine is inactivated vaccines. In this case the virus has been killed using chemicals, heat or radiation and so can’t cause disease. These vaccines teach our immune system how to deal with the virus and our body remembers what to do if it encounters the real thing.
  
  Some seasonal viruses change every year and so we need to update our vaccines at the same rate. For these viruses, like seasonal flu, scientists carry out research into what they expect will be the most common virus in any particular year and create vaccines that contain a mixture of these viruses.

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