I wouldn't put a whole lot of faith in "herd immunity."
First of all, scientists haven't even established that exposure to the virus creates immunity. That's an assumption people make, but there have been numerous cases where people have been sick, got better, and then got sick again.
Also, exposure to the virus creates antibodies to the virus, but the levels of those antibodies vary hugely from patient to patient. It is not yet known what level of antibody, if any, creates immunity. There have been no studies -- much less proof -- that the antibodies give immunity to this particular virus.
We'll have a vaccine long before we'll achieve "herd immunity."