How serious is the coronavirus outbreak? Here is a key number.


Finally, the test positivity rate has become much more reliable nationally in recent weeks. As of the end of March, not all states have reported all negative test results from commercial laboratories. Almost all states now publish these figures.

While our numbers probably don’t capture all of the coronavirus tests in the United States, external evidence now suggests that our data is fairly comprehensive. When the White House Coronavirus Task Force reported the number of tests performed nationwide, its numbers were largely consistent with those of the COVID Tracking Project. In addition, the largest commercial test processors, Quest and LabCorp, have released first-rate statistics that align with ours as part of the COVID tracking project.

The high positivity rate also suggests that new cases in the United States have plateaued only because the country has reached a ceiling in its testing capacity. Looking only at the positives, the United States is headed for 650,000 confirmed cases, but the number of new cases per day seems to level off or even decrease.

Graph of new positive cases of COVID-19 from March 1 to April 15

There are several ways to interpret this evolution. This could suggest, for example, that more than 3.2 million tests done in the United States in the past two months have finally captured a good portion of the people who are actually infected. While it is clear that the country is not catching all cases, the drop in new positive cases may suggest that the country has started to control the spread of the virus.

But there is another way to interpret the decline in new cases: the growth in the number of new tests performed per day has also plateaued. Since April 1, the country has tested around 145,000 people every day without a regular upward trajectory. The growth in the number of new cases per day and the growth in the number of new tests per day are very closely correlated.

Graph comparing the growth of COVID-19 cases and the growth of tests

This close correlation suggests that if the United States tests more people, we would likely see an increase in the number of COVID-19 cases. And combined with the high rate of positivity of the tests, this suggests that the pool of unknown and uncounted cases of COVID-19 across the country is still very large.

Each of these unreported cases is a small tragedy and a microcosm of all the ways the American testing infrastructure still fails. When Sarah Pavis, a 36-year-old engineer in New York, woke up on Tuesday, she was out of breath and her heart was racing. An hour of deep breathing failed to calm her pulse. When her extremities started tingling, she called 911. It was her ninth day of COVID-19 symptoms.

New York’s positivity rate is 55%. More than 111,000 residents of the city have laboratory-confirmed cases of COVID-19, but Pavis is not one of them. When the ambulance arrived at Pavis’ apartment, an EMS worker took his vital signs and then explained that there was little he could do to help. The city’s hospitals admitted only people with blood oxygen levels of 94% or less, he said. Pavis’ blood oxygen reading was 96%. The difference of 2% meant that his illness was not severe enough to warrant hospitalization, not serious enough to be tested, not serious enough to be counted.

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Robinson Meyer is an editor at Atlantic, where it covers climate change and technology.
Alexis C. Madrigal is an editor at Atlantic and the author of Fueling the dream: the history and promise of green technology.


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