The Effects of Flu Vaccines on Responses to Newer Strains

All human viruses were once zoonotic, right? They came from non-human animals. So maybe in those initial years after this bill over, there was a lot of antigenic variation. And if that was the beginning of some immune system, that could have applied the pressure. Maybe then that part of the immune system is no longer needed, but there are other viruses that could still use it. I don't know what you guys think about that.