Background: No remarkable excess mortality attributable to COVID-19 has been observed in Japan until the delta strain of COVID-19 emerged. Object: We sought to quantify high pathogenicity of the delta strain using the National Institute of Infectious Diseases (NIID) model. Method: We applied the NIID model to deaths of all causes from 1987 up through August 2021 for the whole of Japan. Results: Results in Japan show 4105 excess mortality in August 2021 in Japan. It was estimated as 3.8% of the baseline. Discussion and Conclusion: We found substantial excess mortality since the outbreak of COVID-19 had emerged in August 2021, in Japan. It might be due to spread of delta strain at that time.

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