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ObjectivesWe aimed to describe patient characteristics, healthcare utilization, and in-hospital mortality among patients with COVID-19 in Japan across waves.MethodsUsing a large-scale hospital-based database, we identified patients hospitalized for COVID-19 in the first (January-June 2020), second (June-October 2020), third (October 2020-February 2021), fourth (March-June 2021), and fifth (June-December 2021) waves. We summarized patient characteristics, healthcare utilization, and in-hospital mortality during each wave and performed multivariable logistic regression analyses for in-hospital mortality.ResultsFrom the first to fifth waves, the number of patients (mean age ± standard deviation, years) was 2958 (61.2 ± 22.8), 7981 (55.6 ± 25.3), 18,788 (63.6 ± 22.9), 17,729 (60.6 ± 22.6), and 23,656 (51.2 ± 22.3), respectively. There were 190 (6.4%), 363 (4.5%), 1261 (6.7%), 1081 (6.1%), and 762 (3.2%) in-hospital deaths, respectively. The adjusted odds ratios for in-hospital deaths (95% confidence interval) were 0.78 (0.65-0.95), 0.94 (0.79-1.12), 0.99 (0.84-1.18), 0.77 (0.65-0.92), in the second to fifth waves, respectively, compared with the first wave.ConclusionsIn-hospital COVID-19 mortality improved from the first to the second wave; however, during the third and fourth waves, mortality was as serious as in the first wave. Although in-hospital mortality during the fifth wave improved, careful monitoring is needed for upcoming waves, considering changing patient and viral characteristics.

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School of Medicine, University of Tsukuba, Tsukuba, Japan.