Transmission Risk of SARS-CoV-2 Before and After Symptom Onset of the Infector

Kitahara, Mizue; Yamagishi, Takuya; Tsukada, Shota; Sudo, Yasuhiro; Yoshida, Tetsuya; Sunagawa, Tomimasa; Suzuki, Motoi

An outbreak of COVID-19 occurred in Ueda City, Nagano prefecture, Japan with a population of 150,000. Because the residents were a population naïve to COVID-19 and many of them had only one chance of exposure, careful epidemiological investigation could reveal attack rates among close contacts 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 their 20 index cases. During the follow-up period, 17 of them tested positive for SARS-CoV-2 (Overall attack rate 15%) after the exposure to 6 infectors. The median number of close contacts was 5.5 (range 2-14) for the 6 infectors. Attack rates among those 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). Transmission risk of SARS-CoV-2 seems to peak one day before symptom onset, and it was at similar level 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 started after the case identification.