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.

日本长野县上田市爆发了 COVID-19 疫情，人口为 150,000。由于居民对 COVID-19 很幼稚，而且他们中的许多人只有一次接触机会，因此仔细的流行病学调查可以揭示在特定接触日期密切接触者的发病率。我们确定了 89 例病例和 328 名密切接触者。在密切接触者中，114人的20个指示病例只有一次接触机会。在随访期间，其中17人在接触6名感染者后检测出SARS-CoV-2呈阳性（总体发病率为15%）。6 名感染者的密切接触者中位数为 5.5（范围 2-14）。密切接触者的发病率分别为13%（1/8）、20%（2/5）、33%（2/6）、50%（1/2）、64%（4/9和5/5）和 100% (2/2)。SARS-CoV-2 的传播风险似乎在症状出现前一天达到峰值，并且在症状出现前两天（16%）和当天（20%）处于相似水平。除了在发现病例后开始的调查外，还需要采取多学科方法来控制 COVID-19 的爆发。