20% of 328 million is 65.6 million. Divided by 770k is ~85 days.
It’s probably better to think of immunity as regional and also not binary (herd vs not herd).
States like MA and NY have a significant benefit from their population immunity levels, versus FL and TX less so. That is to say they have to do less (if not nothing) to keep their R0 below 1 and whatever cases do show up are less likely to spread widely.
Immunity benefits are cumulative to any policy measures put in place. You can try to slow down the rate (flatten the curve) which might not change the ultimate case count in the end, but can lower deaths through greater availability of care. At some point immunity and mitigation measures combined will get you below R0 of 1.
If you’re an island nation with enough testing for effective contact tracing and constant vigilance and willing to lockdown repeatedly, then the other option is trying to actually prevent any spread even without any immunity, but you have to be able to continue this process indefinitely until a vaccine is widely available.
It’s probably better to think of immunity as regional and also not binary (herd vs not herd).
States like MA and NY have a significant benefit from their population immunity levels, versus FL and TX less so. That is to say they have to do less (if not nothing) to keep their R0 below 1 and whatever cases do show up are less likely to spread widely.
Immunity benefits are cumulative to any policy measures put in place. You can try to slow down the rate (flatten the curve) which might not change the ultimate case count in the end, but can lower deaths through greater availability of care. At some point immunity and mitigation measures combined will get you below R0 of 1.
If you’re an island nation with enough testing for effective contact tracing and constant vigilance and willing to lockdown repeatedly, then the other option is trying to actually prevent any spread even without any immunity, but you have to be able to continue this process indefinitely until a vaccine is widely available.