Summary of studies shows, again, that natural infection confers robust immunity against reinfection. It's time for US Public Health officials to acknowledge prior infection-induced immunity. Any mandates should count previously infected as "immune" along with fully vaccinated.
https://twitter.com/MonicaGandhi9/status/1460672079859109894
Can't get any clearer than this: 0-7% of people become reinfected. Several large-scale studies included here. 
It should be obvious but I'll say it anyway: This does NOT mean if you are unvaccinated that you should go out and try to get infected! This is solely about how those who were infected before vaccines were available (or just who got infected before now) should be designated.
In cases of mandates/passes, how can prior infection be demonstrated? I would have said a + antibody or T-cell test is needed. But the rate of reinfection is so low (regardless of antibodies picked up in tests) that a prior PCR+ test should suffice.
We still don't know how long previous infection-induced immunity will last, or if it will work against other non-Delta strains. Of course, the same question can be asked of fully vaxxed/boosted. But as of now, there is no evidence that, say, March 2020 cases are more PCR+ today.
So to check for population immunity, include both prior infection and vaccination. There are a few tools that do this by state and locality. By state, check here. You can check by state. popimmunity.biosci.gatech.edu
For counties (though does not include vax data), see here: covidestim.org
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