COVID-19 case-clusters and transmission chains in the communities in Japan.

Coronavirus disease 2019 (COVID-19), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), emerged at the end of 2019 and has caused substantial morbidity and mortality in many parts of the world. Clusters of COVID-19 cases associated with superspreading events have often been reported and considered important mechanisms for the spread of the infection.1, 2, 3 In the review article in this journal, Majra et al. summarized the relationship between superspreading events and community spread. To afford collateral evidence, we herein illustrate concrete instances of COVID-19 case-clusters and their transmission chains in the communities in Japan.

Contact tracing to elucidate COVID-19 transmission chains

In Japan, all COVID-19 cases were reported to public health centers of local governments and the Ministry of Health, Labor and Welfare. Contact tracing by public health officers or public health nurses was performed if at all possible, in two directions as follows: 1) backward investigation to figure out the possible source of infection of a case under investigation; this consisted of questions on travel and contact history of 14 days prior to illness onset or infection confirmation by laboratory diagnosis and 2) forward investigation to list possible persons who got infected from a case under investigation; public health officers or public health nurses asked the patient about the contacts encountered within two days leading to the illness onset until infection confirmation. Details of the methodology are available in. ,

Transmission chains of 28 case-clusters in Japan

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 Fig. 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.

Fig. 1

An example of a case-cluster and transmission chains

Transmission dynamics in a case-cluster are illustrated. The blue, green, orange, and yellow boxes represent cases at community superspreading events, cases among co-workers, cases at hospitals/care facilities/schools, and cases among family members, respectively. The three values in square brackets denote the number of patients aged 0–19, 20–59, and 60 or more. Arrows indicate infector-infectee transmission pairs, and different arrow lines mean different sources of infection. A dashed arrow line indicates an indirect transmission chain.

From here, we describe the qualitative findings from the 28 instances of COVID-19 case-clusters. First, superspreading events and places where many people contracted the virus played significant roles in COVID-19 transmission in communities (Supplementary Figs. 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 Figs. 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.

Many transmission chains started from parties, restaurants, or bars (Supplementary Figs. 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 Figs. 5,11,13–15). Heavy breathing and singing could enhance the viral transmission risk via droplets or aerosols. Consecutive superspreading events occurred at such superspreading-prone sites generating a large number of cases in communities (Supplementary Figs. 1–6,13,14). In addition, case-clusters among co-workers were frequently observed (Supplementary Figs. 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 Figs. 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 patients, staff, or visitors visited different facilities, leading to transmissions between those facilities (Supplementary Figs. 9,21–23).

“Spillover” transmissions from hospitals, care facilities, and schools were observed, most of which were household transmissions (Supplementary Figs. 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 Figs. 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 Figs. 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 Figs. 1,10,13,14).

Conclusion

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 Fig. 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. 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.

Fig. 2

Schematic summary of COVID-19 transmission chains

A schematic overview of the common features of COVID-19 transmission chains is illustrated.

Declaration of Competing Interest

None.