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 stated that, in early 2020, they implemented the "cluster-based" approach to curb the spread of coronavirus disease 2019 (COVID-19) by identifying clusters through the retrospective tracing of infection sources. Moreover, they considered this approach a significant contributor to the prevention of outbreaks. However, this claim is not corroborated by existing literature, which consistently describes COVID-19 clusters as comprising a relatively small proportion of the total number of cases detected during this period. This study was aimed to determine whether Japan successfully implemented the cluster-based approach in January-March 2020. METHODS: Publicly available data of 25 clusters (groups of five or more COVID-19 cases that came into contact at a common venue) reported by the Ministry of Health, Labour, and Welfare as of March 31, 2020, were collected. The clusters were classified either retrospectively found or prospectively found. The number of cases related to each cluster was then determined. RESULTS: Among the 1953 COVID-19 cases confirmed until the end of March 2020 in Japan, 4% were related to the three clusters found by retrospective tracing, 45% were related to the 22 clusters found by prospective tracing, and 51% were unrelated to any cluster. CONCLUSIONS: Japan's COVID-19 response in early 2020 did not follow a cluster-based approach. Retrospective tracing has a limited contribution to detecting infectious cases and preventing further transmission. The surveillance system primarily detects non-cluster cases and cluster-related cases using prospective contact tracing.