https://en.m.wikipedia.org/wiki/Herd_immunity
This is settled science based on the past. See link. Herd immunity threshold is a function of how contagious the disease is.
But you need a vaccine to get it there
We don’t have that
Getting sick isn’t “immunity”
I would be hesitant to use the term “settled science,” especially with COVID-19. That’s because COVID-19 seems to be very different than a lot, if not most, other infectious diseases in that such a high percentage of people who are infected are asymptomatic. Therefore calculations of how contagious the disease is, the percentage needed for herd imunity, etc. are very problematic.