Another thing to note is that diseases that kill en masse aren't actually that great of an evolutionary endpoint. To bring up the plague inc mentality, it's entirely possible for a disease to kill more people than it infects, and thus run itself dry. In addition, a severe disease invokes a severe response. Though it's possible for a mild, fast-spreading disease to suddenly develop a lethal version of itself, it's far more likely that more mild versions will appear over time, as diseases with high survival rates are less likely to affect their host's behaviour or spread.This, pretty much. Even without vaccine we will become slowly more resistant to it, until at some point, perhaps 100 or 200 years to the future it has become just another common cold, of barely worth the notice at that point.
The common cold is called that because it's a well-developed version that's mild enough to not kill most sufferers, or to even bother with it. It's just seen as an inevitability of Flu season.