As the rate of new coronavirus cases in the U.S. slows down in some cities, many states and regions are encouraging businesses to open again and ease the lockdowns that have been in place since March. However, public health experts warn that opening up is premature in many of those places.

The U.S. started testing its population for COVID-19 very slowly, but has since ramped up testing in certain regions. By early May the U.S. was performing 200,000 tests a day. Unfortunately, there’s no magic number of tests that is “enough” to contain an outbreak. The most important thing is to test the right people; to evaluate whether that’s happening, public health officials recommend looking at a different number: the percentage of tests coming back positive. This is called the test positivity rate.