Given our decades old focus on cleaning for health, we've lately developed something of a focus on COVID and coronavirus decontamination - both in our ongoing commercial cleaning, and in providing quick response to both clients and the general public in event of a virus exposure in a given facility. Thus, I try to keep abreast of developments of knowledge of the virus, and of its implications. And I think a bit about it.
Here's something I've not seen addressed.
In Arizona, as in most jurisdictions, we approximate numbers of infected individuals, over time, by looking at the most recent percentages of infected in our daily "virus present" test results. Now, those tests - the nasal swabs and so on - report whether the subject has "live" viruses currently in his system, and thus is currently "sick" and able to infect. This has nothing to do with any antibodies he might have in his system. If we see, as we have lately, a 15% infection rate, we conclude that 15% of the population has been infected and will shortly be immune (or dead). We need something like a 60 or 70% percent immune rate for "herd immunity", so we've a long way to go.
But: If I tested negative today, there is no way of knowing (from that test) if I'd had the virus, perhaps with no symptoms, some time ago, and am, while now not sick, immune. There are some unknown number of immune folks out there that we're not counting.
For example, if we assume that an average infection lasts 30 days (likely over-generous), and we've been seeing similar positive results for 4 months (120 days), then, all else being equal, our immune rate, when the current crop recovers, is more like 60%. And thus we're closer to herd immunity than we might think.
The only way to know is to test, randomly, for antibodies. And we're not doing that.