The US tested the wrong people for coronavirus

And you can tell because of a number called the test positivity rate.

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As the rate of new coronavirus cases in the US slows down, many states and cities there are encouraging businesses to open again, easing the lockdowns that have been in place since March. But public health experts warn that in many of those places, opening up is premature.

The reason is that throughout the US, as well as in many other countries, we still don’t really know how many people have the virus, or where they are. That’s dangerous because it means infected people who don’t feel sick are probably mingling with the rest of the population, which could enable further outbreaks. And the only way to really prevent that is by proactively testing people for covid-19 until the people who have it have been tracked down and isolated.

The US started testing its population for covid-19 very slowly, but it’s since ramped testing up, and by early May was performing over 200,000 tests a day. Unfortunately, there’s no magic number of tests that is “enough” to contain an outbreak. The important thing is to test the right people — and to evaluate whether that’s happening, public health officials recommend looking at a different number: The percentage of tests coming back positive. It’s called the test positivity rate.

Note: The headline on this piece has been updated.
Previous headline: What the US needs to do to open up safely

In a previous version of this video, the circles in the infographic at 5:50 and 6:20 were incorrectly sized. The error has been corrected.

Sources:

Our World in Data is a reliable source for country-by-country covid-19 data:


Our World in Data also has weekly-rolling test-positivity rate data which gives a better snapshot of where countries are currently at:


We based our state test-positivity rates on this May 6 Harvard Global Health Institute and NPR analysis:


The Atlantic’s coverage of the importance of test-positivity rates:


An important disclaimer on potential inflation in testing numbers at the national and state level in the US:


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