Risk of intensive care unit admission and mortality in patients hospitalized due to influenza A or B and SARS‑CoV‑2 variants Omicron or Delta.

Background: Respiratory viral infections have significant global health impacts. We compared 30-day intensive care unit (ICU) admission and all-cause mortality risks in patients with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Delta and Omicron variants versus influenza A and B (A/B).

Methods: Data from two retrospective inpatient cohorts in Capital Region of Denmark were analyzed. Cohorts included hospitalized influenza A/B patients (2017-2018) and SARS-CoV-2 Delta/Omicron patients (2021-2022), aged ≥18 years, admitted within 14 days of a positive real-time polymerase chain reaction test result. Cumulative ICU admission and mortality rates were estimated using the Aalen-Johansen estimator. Cox regression models calculated hazard ratios (HRs) for ICU admission and mortality.

Results: The study encompassed 1459 inpatients (Delta: 49%; Omicron: 26%; influenza A: 6.4%; and influenza B: 18%). Cumulative incidence of ICU admission was 11%, 4.0%, 7.5%, and 4.1%, for Delta, Omicron, influenza A, and B, respectively. For ICU admission, adjusted HRs (aHRs) were 3.1 (p < .001) and 1.5 (p = .34) for Delta and Omicron versus influenza B, and 1.5 (p = .36) and 0.71 (p = .48) versus influenza A. For mortality, aHRs were 3.8 (p < .001) and 3.4 (p < .001) for Delta and Omicron versus influenza B, and 2.1 (p = .04) and 1.9 (p = .11) versus influenza A.

Conclusions: Delta but not Omicron inpatients had an increased risk for ICU admission compared to influenza B; however, both variants were associated with higher risks of mortality than influenza B. Only Delta inpatients had a higher risk of mortality than influenza A inpatients.