To categorize counties as high-, medium- and low-risk, you need an adequate amount of testing everywhere. The US testing situation has improved, but it very much varies from location to location.
Some of the most dangerous hotspots in the US right now are probably places where there are few reported positives because there hasn't been much testing ... and the lack of reported positives also hasn't compelled much change in behavior.
Another problem is that counties that have a lot of cases *today* may be on the flat part or even the downslope of their curve, while counties with relatively few cases today may be on the sharpest part of exponential increase.
So the places that look good now may look bad in 2-3 weeks *especially* if they're deemed low-risk and interventions are relaxed. (To some extent, the reverse may be true also and places that were bad before may be bending the curve.) This thing changes *fast*.
A 3-week national lockdown to (i) contain discovered *and undiscovered* simmering hotspots and (ii) get testing volume up to speed everywhere ... and *then* applying recommendations on a county-by-county basis *after* those 3 weeks... that would be a much better policy.
