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?
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Asked by The Ge Ni U S to Andrew, Jade, Jessica, Kevin, Lynn on 22 Jun 2015.
- Jess Wade answered on 22 Jun 2015:
  
  There are so many of me, andrew, Jade Kevin, and Lynn in the world doing great science every day. Scientists working on the flu have been working in labs that have been looking at the flu for years and years. when the flu changes, the virus mutates- a bit of it grows differently, and those start to multiply and take over. We run lots of simulations on the computer to predict what the next year’s viruses might look like- then we make the right vaccines when we have some ideas what they might be.

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