Clinical Outcomes of Hospitalized Patients with SARS-CoV-2 Omicron Variant Vs. Influenza A During Influenza Season 2021 to 2022: A Retrospective Observational Study

Background: Influenza and coronavirus disease 2019 (COVID-19) are respiratory diseases with similar modes of transmission. In December 2021, influenza re-emerged after it had been undetected since March 2020 and the Omicron variant replaced the Delta variant. Data directly comparing the two diseases are scarce. Objectives: To compare the outcomes of patients with both the Omicron variant and influenza during 2021-2022. Methods: We performed a retrospective study conducted in Beilinson hospital, Israel, from December 2021 to January 2022. We included all hospitalized patients with either laboratory-confirmed COVID-19 or influenza. The primary outcome was 30-day mortality. Results: We identified 167 patients diagnosed with Omicron and 221 diagnosed with Influenza A. The median age was 71 years for Omicron and 65 years for influenza. Patients with Omicron had a significantly higher Charlson Comorbidity Index score (4 vs. 3, P < 0.001). Patients with Omicron developed more respiratory failure that needed mechanical ventilation (7% vs. 2%, P = 0.05) and vasopressors (14% vs. 2%, P < 0.001) than patients with influenza. In a multivariate model, 30-day mortality was lower in patients diagnosed with influenza than in patients diagnosed with Omicron (19/221 [9%] vs. 44/167 [26%], hazard ratio 0.45, 95% confidence interval 0.25-0.81). Conclusions: Patients diagnosed with Omicron had higher mortality than patients diagnosed with seasonal influenza. This finding could be due to differences in co-morbidities, the virus pathogenicity, and host responses to infection.