I imagine there are research farms for which immunity is a business and they are interested in “breeding” survivors. But for a commercial farm this wouldnt make economical sense.

In any case all these virus are like the human flu, you can catch them year after year because they mutate.

As someone else posted. Also note that many of these farms dont run the whole business cycle from egg to fully grown chicken. Some farms will be breeders, some will buy the small chicken and are just interested in growing it.