Darwin knew as weaker plants and animals died off stronger strains prospered. Already we hear of booster vaccine shots as the South African variant is not controlled as well as the Wuhan covid strain. Selecting, through pharmacology, would seem to have an end point of either extinction or mass death.

A few questions.. as polio and smallpox were know in antiquity is there advantage in finding a vaccine for a more mature viral strain vs one considered new and novel?
Does a viral strain actually mature or self-extinguish by killing off all hosts?