Read: Don’t believe the COVID-19 models

According to the Tracking Project’s figures, nearly one in five people who get tested for the coronavirus in the United States is found to have it. In other words, the country has what is called a “test-positivity rate” of nearly 20 percent.

That is “very high,” Jason Andrews, an infectious-disease professor at Stanford, told us. Such a high test-positivity rate almost certainly means that the U.S. is not testing everyone who has been infected with the pathogen, because it implies that doctors are testing only people with a very high probability of having the infection. People with milder symptoms, to say nothing of those with none at all, are going undercounted. Countries that test broadly should encounter far more people who are not infected than people who are, so their test-positivity rate should be lower.

The positivity rate is not the same as the proportion of COVID-19 cases in the American population at large, a metric called “prevalence.”* Nobody knows the true number of Americans who have been exposed to or infected with the coronavirus, though attempts to produce much sharper estimates of that figure through blood testing are under way. Prevalence is a crucial number for epidemiologists, in part because it lets them calculate a pathogen’s true infection-fatality rate: the number of people who die after becoming infected.

Read: How to understand your state’s coronavirus numbers

But the positivity rate is still valuable. “It’s not a normal metric, but it can be a very useful one in some circumstances,” Andrews said. The test-positivity rate is often used to track the spread of rare but deadly diseases, such as malaria, in places where most people aren’t able to get tested, he said. And if the same proportion of a population is being tested over time, the test-positivity rate can even be used to calculate the contagiousness of a disease.

Because the number of Americans tested for COVID-19 has changed over time, the U.S. test-positivity rate can’t yet provide much detailed information about the contagiousness or fatality rate of the disease. But the statistic can still give a rough sense of how bad a particular outbreak is by distinguishing between places undergoing very different sizes of epidemics, Andrews said. A country with a 25 percent positivity rate and one with a 2 percent positivity rate are facing “vastly different epidemics,” he said, and the 2 percent country is better off.

In that light, America’s 20 percent positivity rate is disquieting. The U.S. did almost 25 times as many tests on April 15 as on March 15, yet both the daily positive rate and the overall positive rate went up in that month. If the U.S. were a jar of 330 million jelly beans, then over the course of the outbreak, the health-care system has reached in with a bigger and bigger scoop. But every day, 20 percent of the beans it pulls out are positive for COVID-19. If the outbreak were indeed under control, then we would expect more testing—that is, a larger scoop—to yield a smaller and smaller proportion of positives. So far, that hasn’t happened.