We created the live polio vaccine by breeding. That was long before fancy genetic technology. Flu viruses are particularly easy to breed. Viruses have short ge

We created the live polio vaccine by breeding. That was long before fancy genetic technology. Flu viruses are particularly easy to breed. Viruses have short generation times and easy vulnerability to genetic damage. Flu viruses even have a genome that isn't in one chunk, sort of like having chromosomes, making them extra easy to breed. Selection is a simple matter of having it reproduce quickly in the desired cells.