Undetected cases were a major driver of the early spread of SARS-CoV-2 in Wuhan, China, despite being less infectious on a case-by-case basis, according to a modeling study published in the March 16, 2020, online issue of Science.

"The explosion of COVID-19 cases in China was largely driven by individuals with mild, limited, or no symptoms who went undetected," senior author Jeffrey Shaman, professor of environmental health sciences at Columbia University Mailman School, said in a prepared statement.

Before Wuhan, the geographic origin of the current pandemic, enacted travel restrictions and quarantine measures on Jan. 23, 2020, roughly 85% of all infections there were undetected.

In their work, Shaman and his team modeled travel between several hundred cities viral spread from Jan. 10-23, 2020, at a time when awareness of the virus was just beginning to emerge and before travel restrictions had been enacted.

Varying the proportion of four populations – susceptible, exposed, documented infected, and undocumented infected – the authors concluded that undocumented infections were the source of 80% of documented infections in the early phase of the epidemic.

Individually, undocumented infected cases were less infectious than diagnosed, fully symptomatic patients. But in the aggregate, undiagnosed individuals accounted for more of the virus’ spread than documented cases.

The findings lend even greater urgency to the need for extensive social distancing, as well as for ramping up testing in the U.S., where undertesting has been allowed to persist to an extent that is absurd, and very dangerous. Undertesting and/or underreporting is such that the March 15, 2020, situation report of the World Health Organization reported no new cases were diagnosed in the United States in the previous 24 hours.

Globally, on the same day there were almost 11,000 new cases, bringing the total to 153,517.

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