If the flu changes each year , how do scientists create a different flu vaccine each year?

Question: If the flu changes each year , how do scientists create a different flu vaccine each year?
- Keywords:
  flu,
  scientist,
  vaccine
Asked by Emily to Hummy, Lewis, James, Sandra on 12 Mar 2017.
- Lewis Wright answered on 12 Mar 2017:
  
  The vaccine doesn’t prevent the mutation of flu that is going around that year, it is in response to last years mutation, so we are always playing catchup.
  Once the mutation has occurred it takes time to study it and develop a working vaccine, so we are always a year behind.
- Sandra Greive answered on 13 Mar 2017:
  
  Flu viruses mutate quite quickly but usually follow a pattern, which means we can predict what this years flu strain will be based on the the mutations that occurred in last years strain and in the strains in other countries. Sometimes, there is a big change (usually when two different strains combine) this is more difficult to predict. When this happens, the vaccine for that year doesn’t work very well. A bad flu year can also happen when people don’t get vaccinated meaning there is more chance to virus to mutate and spread to people who could get very sick from the flu and its complications e.g. asthmatics.

Question: If the flu changes each year , how do scientists create a different flu vaccine each year?