I'm no expert here, so someone can correct me if I am wrong. But I believe as long as each infected person spreads it to more than 1 other non-infected person the disease will continue to spread until there is a herd immunity level of infections. That spread rate will decrease as more people have antibodies, but it seems unlikely to get below 1 since even all the stay at home orders haven't been able to get that number much below 1.

Sustained spread of the infection stops when percentage of people having immunity is 1-1/R0. So if R0 for Covid-19 is (hypothetically) 3, you need 67% immunity to reach herd immunity. For highly contagious diseases like smallpox that number is very close to 100%

The 1-1/R0 is the point that the virus starts to burn out but there is still a long tail of infections. The total population that is infected will be higher than 1 - 1/R0. Also if the estimates of IFR were too high than the estimates of R0 were too low.

Yes, but there's a world of difference between Rt=1.5 and Rt=2.5, both in the degree of controls necessary to bring Rt close to 1 and in how far it can get out of hand if your controls are somewhat insufficient.

> The closer we get, the better it is. Having 21% "immune" leads to far different outcomes than .1%.

It depends on how long the immunity lasts. If it's permanent, this is indeed great. If it lasts only a few months, this 21% won't make any difference for another wave of infections this fall.

We use the term "immunity" but we should remember that there is a difference between the presence of active antibodies and the presence of immune "memory cells". The latter hang around long-term, even if the former disappear. So at a minimum if one does develop infection they will recover far sooner and with better outcomes. And likely will reach a far lower peak viral load which might bring down transmission.

That is not well established.

It's likely enough, but we don't gain long term immunity to every virus that we successfully clear.

It is well established.

https://en.wikipedia.org/wiki/Memory_T_cell

You are conflating active circulating antibodies with the memory T cells. Which is literally what my entire comment was about.

Of course, there's other confounds. Like the somewhat recent discovery that we end up with antigen experience / "memory" of diseases that we've never contracted. (Perhaps from viral fragments in the environment??)