That famous 'cluster-based' tracing that supposedly saved Japan during the first COVID wave was mostly a myth.
March 24, 2026
Original Paper
Was Japan's cluster-based approach toward coronavirus disease (COVID-19) a fantasy?: Re-examining the clusters' data of January-March 2020
SocArXiv · vuyej_v1
The Takeaway
A re-examination of the 2020 data shows that the 'retrospective tracing' method Japan claimed was its secret weapon actually only accounted for 4% of confirmed cases. More than half of all cases during that period were completely unrelated to any identified cluster.
From the abstract
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 smal