Brehs really banking on herd immunity
The higher the R0 of a disease, the higher thevimmunity needs to be achieved. COVID 19 has a R0 around 5.5 from the studies from Wuhan of a couple months ago . Some other studies say it's around 2.5 so let's say its between 2.5 and 5.5
Rubella and Smallpox have a R0 of 5 to 6 and are both airborne transmitted diseases. The herd immunity for those diseases is from 80 to 85% and that is with vaccination. If we take a R0 of 2.5 then herd immunity is achieved a bit above 50% which means half the population should have antibodies at least (let's not talk about those could maybe fade). The US has had 6M known cases for 350M total population which is less than 2%...
If the world banks on this, we're gonna have a whole lot ot bodies before we achieve immunity. Because without vaccination, it's impossible unless we're planning to stand on loads and loads of cadavers.