Background: As COVID-19 is integrated into existing infectious disease control programs, it is important to understand the comparative clinical impact of COVID-19 and other respiratory diseases.

Methods: We conducted a retrospective cohort study of patients with symptomatic healthcare-associated COVID-19 or influenza reported to the nationwide, hospital-based surveillance system in Switzerland. Included patients were adults (≥18 years) hospitalized for ≥3 days in tertiary care and large regional hospitals. Patients had COVID-19 symptoms and a RT-PCR-confirmed SARS-CoV-2 infection ≥3 days after hospital admission between 1 February 2022 and 30 April 2023, or influenza symptoms and a RT-PCR-confirmed influenza A or B infection ≥3 days after hospital admission between 1 November 2018 and 30 April 2023. Primary and secondary outcomes were 30-day in-hospital mortality and admission to intensive care unit (ICU), respectively. Cox regression (Fine-Gray model) was used to account for time-dependency and competing events, with inverse probability weighting to adjust for confounding.

Results: We included 2901 patients with symptomatic healthcare-associated COVID-19 (Omicron) and 868 patients with symptomatic healthcare-associated influenza from nine hospitals. We found a similar case fatality ratio between healthcare-associated COVID-19 (Omicron) (6.2%) and healthcare-associated influenza (6.1%) patients; after adjustment, patients had a comparable subdistribution hazard ratio for 30-day in-hospital mortality (0.91, 95%CI 0.67-1.24). A similar proportion of patients were admitted to ICU (2.4% COVID-19; 2.6% influenza).

Conclusions: COVID-19 and influenza continue to cause severe disease among hospitalized patients. Our results suggest that in-hospital mortality risk of healthcare-associated COVID-19 (Omicron) and healthcare-associated influenza are comparable.