Incidence Rate of Coronavirus Holding Steady, Not Growing Exponentially

The total number of positive results for the COVID-19 has increased exponentially in the U.S., leading to numerous comparisons with countries like Italy and China that also saw huge growth in the numbers that tested positive for the virus.

But while the total number of positive tests has increased in the last week, those positive results are likely driven by the larger exponential growth in testing for the virus.

The rate of positive results per test—the incidence rate—has consistently hovered around 10 to 12 percent for the past week and a half based on numbers from the COVID Tracking Project. The large number of negative results are indicative of the widespread attention given to the virus.

For more on the difference between incidence and prevalence, see this post.

At the state level, contagion has shown no consistent pattern of exponential growth or decline. Certain states like Arkansas have seen little growth in positive results for infection, whereas states like California and New Jersey saw a mix of growth in infections, stagnation, or even a decline in rate of positive results per test.

Neither the total number of positive results or the incidence rate can give a full understanding as to how prevalent the disease is or how at risk the general population is as they lack demographic and specific geographic details of the population for the tests.

A growing number of total incidents for a disease can be used to indicate a growing epidemic of a disease based on hospital reports from sick patients with substantial symptoms. But the calculus for interpreting those numbers changes when testing for a disease is widespread, with many negative results and many being tested are asymptomatic.

In a recent press conference, former Center for Disease Control and Prevention Epidemic Intelligence Officer Rishi Desai stated that testing for the virus should have started months ago and that the U.S. "should be six times more than where [South Korea] is at."

No Marketable Test Yet

Testing for COVID-19 has become widespread despite no marketable Food & Drug Administration (FDA) approved diagnostic test available. Testing is currently being done through in vitro tests approved through Emergency Use Authorizations (EUAs). EUAs are authorizations to allow for tests to be approved for public use because of public health emergencies without needing the rigorous scrutiny of getting full approval by the FDA.

Thirteen separate tests have been approved since February under EUA because of the public health emergency for COVID-19. Two tests were previously approved for MERS-CoV, in 2013 and 2015 respectively. Without a commercial test available, their EUAs were reissued to extend their availability.

Most of the tests show 100 percent accuracy in detecting the virus. In comparison, rapid influenza tests have sensitivity—the ability to predict positive results—ranging between 54 and 95 percent.

One of the scientists developing a rapid test for Coronavirus recently left the country following an investigation into federally-funded scientists being paid by China through the country's Thousand Talents program.