So how many millions would have to die to get herd immunity of a virus with different strains that its possible to be infected by more than once?
Dunno, you'd have to ask an epidemiologist. It guess it depends on its rate of mutation,my guess is each strain is weaker and weaker as the virus becomes better at entrenching itself in our society. Before long its possible the virus will,
1 rapidly mutate itself out of the "deadly" category becoming endemic. Having a society with more ppl that are hidden carriers, only infecting those who have never encountered it before like children or those with underlying health conditions like old ppl.
2 it can disappear enough where we only have small flare ups here or there, like Ebola or Rabies
3. Or disappear all together after burning itself out like the Spanish Flu, killing as many ppl as possible before doing so.
4. If the virus is around long enough, say decades or centuries, the body will hijack the best and most beneficial aspects of it and mutate itself around it. Example the placenta is the benefit of mammals co existing with a virus. I ASSUME this would only be possible with a stagnant DNA version of the virus and not an unstable RNA (if thats posible) where the body will have time to adapt around it. This would be different from the flu which mutates with a new strain every year.