And we can tell because of one number. Become a Video Lab member! bit.ly/video-lab 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. Sources: Our World in Data is a reliable source for country-by-country covid-19 data: ourworldindata.org/coronavirus-data-explorer?yScale=log&zoomToSelection=true&deathsMetric=true&dailyFreq=true&aligned=true&smoothing=7&country=USA+GBR+CAN+BRA+AUS+IND+ESP+DEU+FRA+RUS Our World in Data also has weekly-rolling test-positivity rate data which gives a better snapshot of where countries are currently at: ourworldindata.org/coronavirus-testing We based our state test-positivity rates on this May 6 Harvard Global Health Institute and NPR analysis: www.npr.org/sections/health-shots/2020/05/07/851610771/u-s-coronavirus-testing-still-falls-short-hows-your-state-doing The Atlantic’s coverage of the importance of test-positivity rates: www.theatlantic.com/technology/archive/2020/04/us-coronavirus-outbreak-out-control-test-positivity-rate/610132/ An important disclaimer on potential inflation in testing numbers at the national and state level in the US: www.theatlantic.com/health/archive/2020/05/cdc-and-states-are-misreporting-covid-19-test-data-pennsylvania-georgia-texas/611935/ Note: The headline on this piece has been updated. Previous headline: What the US needs to do to open up safely Vox.com is a news website that helps you cut through the noise and understand what's really driving the events in the headlines. Check out www.vox.com. Watch our full video catalog: goo.gl/IZONyE Follow Vox on Facebook: goo.gl/U2g06o Or Twitter: goo.gl/XFrZ5H

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