Many people feel they have the virus, but for one reason or another they do not, or could not get tested. Those could be transmitting the infection, and may transmitted to others who will have symptoms (some maybe severe) and will be tested.

Hence, the proportion of individuals infected with SARS-CoV-2 who have minimal symptoms is an important determinant of the pathogen’s pandemic potential, as these infections are likely to go undiagnosed. Investigators in China used mathematical modeling to estimate the number of undocumented infections and their contribution to SARS-CoV-2 transmission in China.

Subjects were categorized into two groups: those with symptoms severe enough to elicit care-seeking and a documented COVID-19 diagnosis, and those with undocumented infections. The model also accounted for changes in human mobility between cities based on recent historic data adjusted for the escalating restrictions on such movement.

At the beginning of the epidemic, the estimated basic reproductive number (R0) was 2.38 and the percentage of undocumented infections was 86.0%. Undocumented infections were estimated to cause 86.2% of all infections. Later in the epidemic and with augmented testing, the proportion of undocumented infections fell to 35%, and the R0 dropped to 1.36 and then to 0.99 as restrictions on geographic movement tightened

Summary Appeared in NEJM Journal watch.

Citation:  Li R et al. Substantial undocumented infection facilitates the rapid dissemination of novel coronavirus (SARS-CoV2). Science 2020 Mar 16; [e-pub]. (