This study used data from multiple sources to infer the proportion of early infections of COVID-19 that went undetected in China and estimate their contribution to virus spread. Their findings help to explain the lightning-fast spread of this virus around the world.

The researchers combined data from Tencent, one of the world’s largest social media and technology companies, with a networked dynamic metapopulation model and used Bayesian inference to analyze early spread of SARS-CoV-2 within China.

They estimate that ∼86% of cases were undocumented before travel restrictions were put in place. Before travel restriction and personal isolation were implemented, the transmission rate of undocumented infections was a little more than half (55%; 95% CI: 46-62%) that of the known cases. However, because of their greater numbers, undocumented infections were the source for 79% of  documented cases. Immediately after travel restrictions were imposed, ∼65% of cases were documented.