"There's no exact number to aim for, but here's a guiding principle: You want a low percentage of your tests to come back positive, around 10% or even lower, sa

"There's no exact number to aim for, but here's a guiding principle: You want a low percentage of your tests to come back positive, around 10% or even lower, says William Hanage, an epidemiologist at Harvard." https://www.npr.org/sections/health-shots/2020/04/22/840526338/is-the-u-s-testing-enough-for-covid-19-as-debate-rages-on-heres-how-to-know https://www.npr.org/sections/health-shots/2020/04/22/8405263... There are several other sources out there that state that for example the WHO also recommended it as a guideline. Yes not everyone is being tested but the 10% does account to some degree of the "severity" of those being tested. If you're only able to test the most likely cases you'll get a much higher % positive.