Undocumented Coronavirus Infections may Be More Numerous than Was Thought

During the initial outbreak in China, 79% of documented infections may have originated from undocumented cases, according to a study published in Science.

We use observations of reported infection within China, in conjunction with mobility data, a networked dynamic metapopulation model and Bayesian inference, to infer critical epidemiological characteristics associated with SARS-CoV2, including the fraction of undocumented infections and their contagiousness.

Li, R., Pei, S., Chen, B., Song, Y., Zhang, T., Yang, W., & Shaman, J. (2020). Substantial undocumented infection facilitates the rapid dissemination of novel coronavirus (SARS-CoV2). Science, eabb3221. doi: 10.1126/science.abb3221

The study also estimates that 86% of all infections in China were undocumented prior to the imposition of travel restrictions at the end of January.

These numbers are derived from mathematical modelling, and it is not possible to know for certain how accurate they are, or whether they still hold true outside of China. If the study is correct, this suggests the spread of the virus will be particularly challenging to contain, as undocumented (and often asymptomatic) cases are responsible for most of the transmission. The study also highlights the importance of limiting social contact, regardless of symptoms.

While these findings are disheartening, a positive interpretation is also possible: if the total number of cases is higher than originally thought, this would mean that the overall mortality rate has been overestimated.