[100ideas]

Just to spell it out: herd immunity depends on sustained immunity postinfection. It seems possible, perhaps probable, that covid19 "immunity" may have an unusually short duration or attenuated response, resulting in an effective herd population that is a fraction of the theoretical herd population. I.e. if 60% of Americans have been infected and recovered after 12 months, but antibody levels drop 25% after 12 months, the true number of immune americans comprising the herd will be less than 60%.

If so, we may need to consider pursuing a relatively slow and steady relaxation of lockdown to tune the rate of new infections such that herd immunity is kept as stable as possible - i.e. flatten curve into truly flat-but-nonzero line