Instead of killing the chickens why don’t they give them a vax?..................
What is they one day decide to do this to humans?..................
They don’t vaccinate the chickens because of the fact that vaccination is not a shield against disease, but an immune system training aid.
If a chicken has a weak immune response following vaccination, it might still be strong enough for the chicken to survive catching HPAI. If a chicken has HPAI which is (mostly) controlled by the immune system, it can still shed virus. That virus can get into wild bird populations because there is no way to keep outdoor poultry and wild birds away from each other. The wild birds then go and infect other poultry flocks. They can also infect pigs, which act as mixing bowls for the creation of new viruses. So the preferred method of controlling HPAI (or even LPAI) outbreaks is to cull the affected birds.
The scientific community is well-aware of the fact that vaccines are not shields against disease and that individual immune response to vaccines is variable. But we don’t cull humans. We use other methods to isolate the infected. Try telling a chicken to self-isolate for several days...