Estimation of the onset ratio and the number of asymptomatic patients of COVID-19 from the proportion of untraceable patients

A simple method is devised to estimate the onset ratio of COVID-19 patients from the proportion of untraceable patients tested positive, which allows us to obtain the number of asymptomatic patients, the number of infectious patients and the effective reproduction number. The recent data in Tokyo indicate that there are about six to ten times as many infectious patients in the city as the daily confirmed new cases. It is shown that, besides social distancing and use of effective masks, a quarantine measure on non-symptomatic patients is critically important in controlling the pandemic.