Was Japan’s cluster-based approach toward coronavirus disease (COVID-19) a fantasy?: Re-examining the clusters’ data of January–March 2020

BACKGROUND The Japanese government and experts have claimed that they adopted the “cluster-based” approach to suppress the spread of novel coronavirus disease 2019 (COVID-19) in early 2020 by detecting clusters through the retrospective tracing of infection sources. Moreover, they considered this approach to be a significant contributor in the prevention of outbreaks. However, this claim is not corroborated by existing literature, which consistently described COVID-19 clusters as comprising a relatively small proportion of the total number of cases detected during the period. METHODS To determine whether Japan successfully implemented the cluster-based approach in January–March 2020, the author collected publicly available information regarding 25 clusters (groups of five or more COVID-19 cases that came in contact at a common venue) reported by the Ministry of Health, Labour, and Welfare as of March 31, 2020. These clusters were classified into retrospectively or prospectively reached using a tracer. The number of cases related to each cluster was determined. RESULTS Among 2135 COVID-19 cases confirmed until the end of March 2020 in Japan, 9.3% of the cases were related to the five clusters found by retrospective tracing. In contrast, 35.3% of the cases were related to the 20 clusters found by prospective tracing, while 55.4% were unrelated to any cluster. CONCLUSIONS Japan’s COVID-19 response in early 2020 was not cluster based. Retrospective tracing has a limited contribution in detecting infectious cases and preventing further transmissions. The surveillance system mainly detects non-cluster cases and cluster-related cases using prospective contact tracing.