[StavrosK]

I think it's easier to understand when you take it to the extremes: Assume nobody has the thing you're testing for. You test 100k people at 99.9% specificity, which means you get 1k positives just because of the rate. Since nobody hass the thing you're testing for, they're all false.

When the thing you're testing for is very rare, it's just as rare that the people who tested positive will actually have it.

[iso947]

But we know there are more than 1% cases.

We have good data that the IFR is in the 0.1-1% range, putting cases in say MA in the 7% range a couple of weeks ago (time from infection to death), which based on confirmed cases would put it well above 10% now

That means you’d have 1k false positive and 10k true positive from a test.