Evaluation of characteristics and prognosis of COVID-19 patients requiring invasive mechanical ventilation during dominance of nonvariant, alpha, delta, and omicron variants in tertiary hospitals of Japan

In November 2021, the B.1.1.529 (omicron) variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was detected in South Africa and subsequently rapidly spread around the world. Despite the reduced severity of the omicron variants, many patients become severely ill after infection and undergo invasive mechanical ventilation, but there are few reports on their background and prognosis throughout all variant periods. This study aimed to evaluate risk factors affecting patients requiring invasive mechanical ventilation with each variant of COVID-19 pandemic in Japan from nonvariants to omicron variants. This retrospective observational study was conducted at the Department of Emergency and Critical Care Medicine, Kansai Medical University Hospital and Kansai Medical University Medical Center, Osaka, Japan, from March 2020 to March 2023. Eligible patients were those who underwent invasive ventilation for COVID-19 pneumonia. We set the primary endpoint as in-hospital mortality. Multivariable logistic regression analysis adjusted for clinically important variables was performed to evaluate the clinical outcomes. We included 377 patients: 118 in the Nonvariant group, 154 in the Alpha group, 42 in the Delta group, and 63 patients in the Omicron group. Mortality rates for each group were 23.7% for the Nonvariant group, 12.3% for the Alpha group, 7.1% for the Delta group, and 30.5% for the Omicron group. Patient age was significantly associated with increased mortality (adjusted odds ratio [AOR]: 1.097; 95% confidence interval [CI]: 1.057-0.138, P