Many epidemiologists believe that the initial COVID-19 infection rate was undercounted due to testing issues, asymptomatic and alternatively symptomatic individuals, and a failure to identify early cases. A study from Penn State estimates that the number of early COVID-19 cases in the U.S. may have been more than 80 times greater and doubled nearly twice as fast as originally believed.
In a paper published in Science Translational Medicine, researchers estimated the detection rate of symptomatic COVID-19 cases using the Centers for Disease Control and Prevention’s influenza-like illnesses (ILI) surveillance data over a three-week period in March 2020.
“We analyzed each state’s ILI cases to estimate the number that could not be attributed to influenza and were in excess of seasonal baseline levels,” said Justin Silverman, assistant professor in Penn State’s College of Information Sciences and Technology and Department of Medicine. “When you subtract these out, you’re left with what we're calling excess ILI – cases that can't be explained by either influenza or the typical seasonal variation of respiratory pathogens.”
The researchers found that the excess ILI showed a nearly perfect correlation with the spread of COVID-19 around the country. Said Silverman, “This suggests that ILI data is capturing COVID cases, and there appears to be a much greater undiagnosed population than originally thought.”
Remarkably, the size of the observed surge of excess ILI corresponds to more than 8.7 million new cases during the last three weeks of March, compared to the roughly 100,000 cases that were officially reported during the same time period.
