Factors contributing to the sharp rise in excess mortality in Japan since 2021

Since 2021, excess mortality in Japan has risen sharply. This study adopts an ecological approach to investigate correlations between prefectural excess mortality and factors such as demographic structure, income, medical capacity, vaccination rates, and life expectancy. The primary contributing factors identified include population aging, COVID-19 mortality, a low ratio of habitable land, a high rural population ratio, and physician shortages. In mountainous and depopulated regions, fragile medical infrastructure—worsened by depopulation, aging, and limited access to healthcare—collapsed under the pressure of the SARS-CoV-2 outbreak, contributing to elevated excess mortality. While SARS-CoV-2 vaccination initially helped suppress excess mortality until June 2022, a positive correlation has emerged since July. This may be attributable to immune imprinting caused by original strain vaccines, which results in insufficient production of neutralizing antibodies against the Omicron spike protein expressed by Omicron-based vaccines and may contribute to heightened spike protein toxicity. A shift away from urban-centric health policies and vaccine-only strategies toward multifaceted approaches aimed at strengthening herd immunity is urgently needed.