This is a link to an article that explains why "immunity passports" won't be any good for quite some time unless you live in an area where there has been a lot of infection in the general public. It's just simple epidemiology, but most people don't understand it. This guy lays it out pretty well. He does calculate positive predictive value incorrectly, coming up with 49% for his answer when if he had calculated it properly, it would have come out to 51%, but the point that it's "no better than a coin toss" still stands, even after the error is corrected.
In essence, the point of it is that the positive predictive value of any test depends on the prevalence of the condition in the population being tested.
https://www.medscape.com/viewarticle...=2404654&faf=1
"We are not provided with wisdom, we must discover it for ourselves, after a journey through the wilderness which no one else can take for us, an effort which no one can spare us, for our wisdom is the point of view from which we come at last to regard the world." --M. Proust