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How Many COVID-19 Cases May Be Going Unreported, According To New Modeling Study


Tom Hale

Senior Journalist

clockApr 13 2020, 16:16 UTC

MONTREAL in QUEBEC, CANADA 03 24 2020: People queue for a COVID-19 test screening at the walk-in clinics in downtown Montreal. meunierd/Shutterstock

How many COVID-19 cases are going undetected? A new study has attempted to gain a more realistic picture of the novel coronavirus outbreak by using mathematical modeling to estimate the number of unreported cases of COVID-19 in early March. 

Reporting in the journal Infection Control and Hospital Epidemiology, mathematicians from Washington University in St. Louis and Augusta University in Georgia looked at the number of confirmed cases in an area by March 9 (when data was available up to), then used a mathematical model to take into account an area’s "transmission probability," which looks at the area population density, the portion of the population living in close urban areas, and important demographics, such as age. They even integrated data looking at new emerging information about how long the virus survives on a variety of surfaces and in the air. 


By their workings, Italy reported 1 case for every 4 unreported cases as of March 9, 2020, when it had the highest cases in Europe. South Korea was also estimated to be reporting 1 case for about every 4 actual cases as per this time. In Germany, the ratio was as low as 1 to 3 cases, while France had a ratio of 1 to 5 cases, and Iran had 1 in 34 reported.

In China, where the outbreak first originated, the researchers argued there could be a much bigger difference between confirmed cases and unreported cases. Given the size of China’s population, they produce a range varying from 1 in 149 to 1 in 1,104 reported cases to those unconfirmed. That means the modeling predicts between 12 million to 89 million COVID-19 cases might not have been reported in China alone by early March.

The study authors note that the US had insufficient data at the time to reach any concrete conclusions. However, their preliminary results based on the limited data available for then suggests the US had perhaps as many as 90,000 cases as of March 9, but had only reported around 500 cases at the time. 


"We wanted to provide info on the real magnitude of the problem, not just the tip of the iceberg," Dr Arni S R Srinivasa Rao, director of the Laboratory for Theory and Mathematical Modeling in the Division of Infectious Diseases at the Medical College of Georgia at Augusta University, said in a statement.

“With better numbers, we can better assess how long the virus will persist and how bad it will get. Without these numbers, how can health care systems and workers prepare for what is needed?"

Of course, all of these figures are just predictions and estimates based on data, not confirmed numbers. Most crucially, the estimates only looked for the number of under-reporting before the first peak of COVID-19 up to the date of March 9, so the findings might not necessarily account for many of the events or trends over the past month or so. Dr Rao also notes the accuracy of reported cases likely has improved since March 9 thanks to the increased availability of reliable testing and governments eventually rolling out such testing. 


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