Difference in clinical courses and causes of COVID-19-related deaths in hospitalized patients infected with omicron and delta variants: A retrospective study in Japan.

Despite the lower rate of severe illness associated with the omicron variant than the delta variant, more deaths have occurred among patients with mild-to-moderate COVID-19 in Japan since the omicron variant surge during the sixth wave. This study aimed to elucidate the background, clinical course, and causes of death in patients with COVID-19. We conducted a retrospective observational study on patients with COVID-19 admitted to the National Center for Global Health and Medicine who subsequently died during the delta (July-September 2021) and omicron variant outbreaks (December 2021-August 2022). Among the 20 patients who died during the delta variant epidemic, the main causes of death were pneumonia ( = 16, 80%), preadmission complications ( = 3, 15%), and complications occurring during hospitalization ( = 1, 5%). However, during the omicron variant epidemic, 7/24 patients (29%) died of pneumonia, 11 (46%) died of complications before admission, and 6 (25%) died of complications during admission. During the omicron variant outbreak, two-thirds of the COVID-19 deaths during hospitalization were not primarily caused by pneumonia, unlike the delta variant outbreak, during which pneumonia had a greater impact on mortality. As patient demographics and clinical pictures change, the establishment of medical infrastructure for patients with life-threatening comorbidities and careful monitoring of acute COVID-related complications are essential.