How can make the correct flu vaccine when viruses can be different every year?

Question: How can make the correct flu vaccine when viruses can be different every year?
- Keywords:
  disease,
  vaccine,
  virus
Asked by owlie to Frank, Leila on 22 Mar 2013.
- Leila Nichol answered on 22 Mar 2013:
  
  The “flu vaccine” also known as influenza vaccination, is an annual vaccination using a vaccine specific for a given year to protect against the highly variable flu virus. Each seasonal flu vaccine contains antigens representing three or four flu virus strains: one influenza type A subtype H1N1 virus strain – which you might have heard of this is the common winter flu virus, and then type A subtype H3N2, and type B virus strains. But you’re right – viruses had become immune to vaccines and a more potent type can occur, then we have to find a vaccine for this one!

Question: How can make the correct flu vaccine when viruses can be different every year?