C oronavirus disease (COVID-19) typically causes febrile illness with respiratory symptoms (1,2), and many countries worldwide have been affected. Before characterizing COVID-19 as a pandemic in March 2020 (3), the World Health Organization advised countries to take measures to reduce spread of the virus, including identifying cases and clusters, isolating patients, tracing contacts, and preventing community transmission (4). Several countries have reported on the characteristics of a small number of clusters of COVID-19 cases (5,6). However, few comprehensive reports provide an overview of clusters of COVID-19 cases in communities and the significance of such clusters. We analyzed 61 COVID-19 clusters among various communities in Japan and identified 22 probable primary cases that might have contributed to the disease incidence in clusters. The Study We analyzed COVID-19 cases in Japan reported during January 15-April 4, 2020. All COVID-19 cases confirmed by reverse transcription-PCR in Japan must be reported to the Ministry of Health, Labour and Welfare. Through case interviews, local health authorities collected demographic and epidemiologic information, such as possible source of infection and contact and travel history. During the study period, a total of 3,184 laboratory-confirmed COVID-19 cases, including 309 imported cases, were reported. Among cases of local transmission, 61% (1,760/2,875) had epidemiologic links to known cases (Figure 1, panel A). We excluded 712 cases detected on a cruise that was anchored at Yokohama Port, Japan, from February 3 through March 1 (7). We defined a cluster as >5 cases with primary exposure reported at a common event or venue, excluding within-household transmissions. Our definition also excluded cases with epidemiologic links to secondary transmission. For example, in the following scenario we would exclude cases A and B: boy A is a friend of boy B whose grandmother C contracted nosocomial COVID-19 in a nursing home from which ≥5 cases were reported; although all 3 have symptoms develop and are diagnosed with COVID-19, we would consider only grandmother C part of a cluster from the nursing home. By investigating the epidemiologic links among cases, we identified 61 COVID-19 clusters in various communities. We observed clusters of COVID-19 cases from 18 (30%) healthcare facilities; 10 (16%) care facilities of other types, such as nursing homes and day care centers; 10 (16%) restaurants or bars; 8 (13%) workplaces; 7 (11%) music-related events, such as live music concerts, chorus group rehearsals, and karaoke parties; 5 (8%) gymnasiums; 2 (3%) ceremonial

Background: As the COVID-19 pandemic spread, the Japanese government declared a state of emergency on April 7, 2020 for seven prefectures, and on April 16, 2020 for all prefectures. The Japanese Prime Minister and governors requested people to adopt self-restraint behaviors, including working from home and refraining from visiting nightlife spots. However, the effectiveness of the mobility change due to such requests in reducing the spread of COVID-19 has been little investigated. The present study examined the association of the mobility change in working, nightlife, and residential places and the COVID-19 outbreaks in Tokyo, Osaka, and Nagoya metropolitan areas in Japan. Methods: First, we calculated the daily mobility change in working, nightlife, and residential places compared to the mobility before the outbreak using mobile device data. Second, we estimated the sensitivity of mobility changes to the reproduction number by generalized least squares. Results: Mobility change had already started in March, 2020. However, mobility reduction in nightlife places was particularly significant due to the state of emergency declaration. Although the mobility in each place type was associated with the COVID-19 outbreak, the mobility changes in nightlife places were more significantly associated with the outbreak than those in the other place types. There were regional differences in intensity of sensitivity among each metropolitan area. Conclusions: Our findings indicated the effectiveness of the mobility changes, particularly in nightlife places, in reducing the outbreak of COVID-19.

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The overall coronavirus disease secondary attack rate (SAR) in family members was 19.0% in 10 prefectures of Japan during February 22–May 31, 2020. The SAR was lower for primary cases diagnosed early, within 2 days after symptom onset. The SAR of asymptomatic primary cases was 11.8%.

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