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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- Royal Society of Chemistry: Find out more on:
  
  disease
  
  medicine
  
  microorganism
Asked by to Kieren, Rowena, Roy on 19 Mar 2014. This question was also asked by .
- Rowena Fletcher-Wood answered on 19 Mar 2014:
  
  They don’t – they have to race against the virus trying to keep up with it! Sometimes the viruses adapt a lot and are more resilient than other times. When a new dangerous strain comes out, scientists can only act to make the vaccine against it once it’s seen.

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