ABSTRACT
An outbreak of coronavirus 2019 (COVID-19) occurred in Ueda City, Nagano prefecture, Japan, which has a population of 150,000. The residents were a population naïve to COVID-19, and many of them had only one chance of exposure, in which careful epidemiological investigation could reveal attack rates among close contact on the specific date of exposure. We identified 89 cases and 328 close contacts. Among the close contacts, 114 had only one chance of exposure to the 20 index cases. During the follow-up period, 17 close contacts tested positive for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), overall attack rate of 15%, after the exposure to 6 infectors. The median number of close contacts for the 6 infectors was 5.5 (range 2-14). Attack rates among these close contacts were 13% (1/8), 20% (2/5), 33% (2/6), 50% (1/2), 64% (4/9 and 5/5), and 100% (2/2), respectively. The transmission risk of SARS-CoV-2 appears to peak one day before symptom onset, and is at similar levels two days before (16%) and on the day (20%) of symptom onset. A multidisciplinary approach is needed to control the COVID-19 outbreak, in addition to investigation, which began after case identification.
