I don't know specifically about COVID but I'll share some old information I remember from my university days.

Basically it's not in a virus's best interests to kill their host. They need you to stay alive so that they can use you to continue reproducing so what ends up happening is overtime a virus will evolve to be less harmful to it's host organism. From what I understand the Spanish Flu never actually ended it just evolved to stop killing it's host as much. Strains that descended from the original pandemic are now just a normal part of flu season.