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.

3.3 Subgroup analysis In the subgroup analysis, patients who needed oxygen support but not mechanical ventilation in the months of waves 2 and 3 had significantly lower aORs for in-hospital mortality, while those who needed mechanical ventilation did not show such a significant trend ( Table 3 ).

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