How much do mild, undocumented cases of COVID-19 contribute to the spread of the disease?

Bottom Line:

Before travel restrictions were placed, the majority of COVID-19 infections in China were undocumented, as they were associated with mild, limited or no symptoms. Undocumented cases appear to play the largest role in dissemination of COVID-19.


Li, R. et al. Substantial undocumented infection facilitates the rapid dissemination of novel coronavirus (SARS-CoV2). Science (2020). DOI: 10.1126/science.abb3221

Date Published:

16 March 2020


In this study, mathematical modelling was used to analyze the rates and infectiousness of undocumented cases of coronavirus in China from January 10-23, a time period before travel restrictions were placed. They estimated that 86% of all infections were undocumented before travel restrictions were placed on January 23, 2020. They estimated that these undocumented cases were half (55%) as infectious as documented cases, but because of their high prevalence, it was estimated that they were the infection source of 79% of the documented cases. This highlights that asymptomatic carriers are a key source of the infection spreading in the population. Therefore, this study supports public health guidelines supporting social distancing, regardless of whether or not people are experiencing symptoms.

Summary by: Megan Wheatley