key: cord-0728018-wvzr4his authors: Furuse, Yiiki; Tsnchiya, Naho; Miyahara, Reiko; Yasuda, Dckoh; Sando, Eiichiro; Ko, Yura K; Imainura, Takeaki; Morimoto, Konosiike; Iinamura, Tadatsugu; Shobugawa, Yugo; Nagata, Shohei; Tokiimoto, Atsuna; Jiiidai, Kazuaki; Suzuki, Motoi; Oshitani, Hitoshi title: COVID-19 case-clusters and transmission chains in the communities in Japan date: 2021-08-11 journal: J Infect DOI: 10.1016/j.jinf.2021.08.016 sha: bd592c413ac9a9ad3fba9871b46075247369ae4e doc_id: 728018 cord_uid: wvzr4his nan Yiiki Furuse a,1,* , Naho Tsnchiya b,1 , Reiko Miyahara c,d,1 , Dckoh Yasuda e,f,1 , Eiichiro Sando e,f,1 , Yura In Japan, all COVID-19 cases were reported to public health centers of local governments There were 213 case-clusters with five or more associated cases between January and July 2020 in Japan. Of these, we report 28 instances in which more than 10 cases were associated; the transmission was sustained for at least three transmission-generations, and detailed information about transmission events and venues was available. For example, consider the instance in Figure 1 . Backward contact tracing enabled a public health authority to notice that several COVID-19 patients had attended party A before illness onset, thereby identifying this party as a possible source of the SARS-CoV-2 infection. Subsequently, the rest of the party participants were tested, and more persons were detected positive for the virus. A patient from party A transmitted the virus to one's co-worker, and another patient from the party spread the infection to three more people at dinner C. A patient that contracted the virus at dinner C further transmitted the virus to one's family member. Concurrently, a nosocomial outbreak of SARS-CoV-2 occurred in hospital B. Through backward contact tracing, it was discovered that a patient in the hospital had an epidemiological link with party A. There was further transmission to a family member of a patient in hospital B, and the one was the last case related to the cluster. From here, we describe the qualitative findings from the 28 instances of COVID-19 caseclusters. First, superspreading events and places where many people contracted the virus played significant roles in COVID-19 transmission in communities ( Supplementary Figures 1-28) . It is worth noting that the first case confirmed by laboratory diagnosis in a case-cluster was different from the case with the earliest illness onset in the cluster in 12 instances ( Supplementary Figures 1-3,7,12,13,15,19,21-23,28) . This underscores the power of backward contact tracing to detect superspreading events and places. Identification of superspreading events and places led to the detection of more associated cases and their contacts. Consequently, transmission chains were blocked off, and the infection spreads were contained within five weeks for all 28 instances. Figures 1-12) . Eating and drinking together may have increased the chance of viral transmission because many people gather, chat for a long time in close proximity, and do not wear a facemask. Gymnasiums and music-related events are also common places for superspreading ( Supplementary Figures 5,11,13-15 ). Heavy breathing and singing could enhance the viral transmission risk via droplets or aerosols 7 . Consecutive superspreading events occurred at such superspreading-prone sites generating a large number of cases in communities ( Supplementary Figures 1-6,13,14) . In addition, case-clusters among co-workers were frequently observed (Supplementary Figures 2,3 ,5,7,12,15-18). However, it is difficult to discern if the virus was transmitted among co-workers at the workplace or during social interactions. Many COVID-19 cases were also ascertained in hospitals, care facilities, and schools including nurseries (Supplementary Figures 1,7-11,15,19-28) . Given that these are the areas where the same people stay or gather every day, it is unknown whether one or a few superspreading events occurred or long sequential transmission chains existed. Frequent and close contacts and the presence of vulnerable people in those places could prolong the outbreaks and make the case-clusters very large. In some cases, the same patient, staff, or visitors visited different facilities, leading to transmissions between those facilities (Supplementary Figures 9,21-23 ). -Spillover‖ transmissions from hospitals, care facilities, and schools were observed, most of which were household transmissions (Supplementary Figures 1,8,10,19-26) . Outbreaks in hospitals, care facilities, and schools rarely led to community superspreading events such as those at parties. Conversely, we noticed the introduction of infections into hospitals, care facilities, and schools in some instances by cases from community superspreading events (Supplementary Figures 1,7-11,15 ). Although most household transmissions were observed at the edge of transmission chains, they could sometimes lead to further spread outside the household to communities or healthcare/care facilities ( Supplementary Figures 3,12,14,19 ,24). As described earlier, transmissions to hospitals, care facilities, schools, and family members are generally located at the edge of transmission chains. Therefore, the infections in children and older adults were usually observed at a later stage of local spread. This corroborates the previous findings that people in their 20s-50s may be the driving force of the COVID-19 epidemic [7] [8] [9] . Nevertheless, because of the super-aged society in Japan, it was also observed that community superspreading took place from the resident community of older adults and the transmissions were sustained among people in that age group (Supplementary Figures 1,10,13,14) . This study showed examples of how SARS-CoV-2 was transmitted in communities and described the common features of COVID-19 transmission chains as schematized in Figure 2 . Through our investigation, we figured out that -three Cs‖-closed spaces with poor ventilation, crowded places with many people nearby, and close-contact settings-were the conditions leading to a high risk of viral transmission 10 . Because the epidemiological investigation was conducted by interviews and relied on the voluntary cooperation of patients, there could be missed transmission chains. Still, the combination of backward and forward contact tracing enables us to understand the mechanisms of transmission dynamics and prevent the further spread of the infection. Identifying and Interrupting Superspreading Events-Implications for Control of Severe Acute Respiratory Syndrome Coronavirus 2 SARS-CoV-2 (COVID-19) superspreader events Cluster-based approach to Coronavirus Disease 2019 (COVID-19) response in Japan COVID-19 Health system response monitor: Japan. WHO Institutional Repository for Information Sharing Clusters of coronavirus disease in communities Epidemiology and transmission dynamics of COVID-19 in two Indian states Resurgence of SARS-CoV-2: Detection by community viral surveillance