If there was a quick and reliable way for people at this point to determine if they had already been exposed to the virus and had built up an immune response, I think we find that not only is exposure much broader in the population than we realize, but a whole lot of people who are being asked to stay home would find they could go back to work.

Of course that assumes that covid-19 is like other viral and bacterial infections which leave a footprint where an infected person acquires an immune response robust enough to prevent future infection. But as the prevalence of exposure increases, it seems knowing the number exposed will be far more important epidemiologically than knowing the number who currently test positive.