Undetected cases, many of which were likely not severely symptomatic, were largely responsible for the rapid spread of the COVID-19 outbreak in China, according to new research by Columbia Mailman School of Public Health. The findings based on a computer model of the outbreak are published online in the journal Science. A press conference was held at noon on March 16^th.

Findings:

• 86% of all infections were undocumented prior to the Jan. 23 Wuhan travel shutdown
• Per person, these undocumented infections were 52% as contagious as documented infections yet were the source of 2/3 of documented infections
• Government control efforts and population awareness have reduced the rate of spread in China; after travel restrictions and control measures were imposed, it spread less quickly

“The explosion of COVID-19 cases in China was largely driven by individuals with mild, limited, or no symptoms who went undetected,” says co-author Jeffrey Shaman, PhD, professor of environmental health sciences at Columbia Mailman School. “Depending on their contagiousness and numbers, undetected cases can expose a far greater portion of the population to virus than would otherwise occur. We find for COVID-19 in China these undetected infected individuals are numerous and contagious. These stealth transmissions will continue to present a major challenge to the containment of this outbreak going forward.”

The researchers used a computer model that draws on observations of reported infection and spread within China in conjunction with mobility data from January 10-23 and January 24-February 8. They caution that major changes to patient documentation practices or rapid developments on travel restrictions and control measures, may make predictions difficult.

Read more in Science.