A New Statistic Reveals Why America’s Coronavirus Numbers Are Flat
We need a thorough test regime. Prevalence vs positivity are key data for decision making.
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.
In an ideal testing regime—and in any of the testing regimes that experts say must exist before the United States can end its lockdowns—anyone with a fever and a dry cough would be tested immediately. A very large portion, if not most, of those people would turn out not to be infected with the new coronavirus, SARS-CoV-2, because humans are susceptible to many other respiratory infections. But when tests are rationed so strictly, only people with severe symptoms make it into the testing pool, ensuring that the positivity rate will be extremely high.
…But there is another way to interpret the decline in new cases: The growth in the number of new tests completed per day has also plateaued. Since April 1, the country has tested roughly 145,000 people every day with no steady 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 tightly correlated.
This tight correlation suggests that if the United States were testing more people, we would probably still be seeing an increase in the number of COVID-19 cases. And combined with the high test-positivity rate, it suggests that the reservoir of unknown, uncounted cases of COVID-19 across the country is still very large.