Temporal trends in clinical characteristics and in-hospital mortality among patients with COVID-19 in Japan for waves 1, 2, and 3: A retrospective cohort study

INTRODUCTION: Little information is available on the temporal trends in the clinical epidemiology and in-hospital mortality of patients with coronavirus disease 2019 (COVID-19) in Japan for waves 1, 2, and 3. METHODS: A national claims database was used to analyze the time trends in admission, medical procedure, and in-hospital mortality characteristics among patients with COVID-19. Patients who were ≥18 years and discharged from January 1, 2020 to February 28, 2021 were included. RESULTS: A multilevel logistic regression analysis of 51,252 patients revealed a decline in mortality in waves 2 and 3 (risk-adjusted mortality range = 2.17–4.07%; relative risk reduction = 23–59%; reference month of April 2020 = 5.32%). In the subgroup analysis, a decline in mortality was also observed in patients requiring oxygen support but not mechanical ventilation (risk-adjusted mortality range = 5.98–11.68%; relative risk reduction = 22–60%; reference month of April 2020 = 15.06%). Further adjustments for medical procedure changes in the entire study population revealed a decrease in mortality in waves 2 and 3 (risk-adjusted mortality range = 2.66–4.05%; relative risk reduction = 24–50%). CONCLUSIONS: A decline in in-hospital mortality was observed in waves 2 and 3 after adjusting for patient/hospital-level characteristics and medical treatments. The reasons for this decline warrant further research to improve the outcomes of hospitalized patients.