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:
  disease,
  flu,
  medicine,
  vaccine,
  virus
Asked by anon-248496 to Zoya, Tom, Stacey, Laura, James, Connor on 15 Mar 2020.
- James Lees answered on 15 Mar 2020:
  
  It’s an educated guess.
  
  Scientists use lots of data to figure out which flu strains are going to be the most common that year and turn that into a vaccine that they use.
- Zoya answered on 18 Mar 2020:
  
  The flu vaccine is normal a mixture of a few strains of viruses that are most prevalent. The benefit of giving you a mixture dose is so you have sort of over all immunity. Flu vaccines are strains of influenza.

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