Rates of in-hospital death and intensive care unit (ICU) admissions were similar among adults hospitalized in Switzerland with healthcare-linked COVID-19 or influenza infections during SARS-CoV-2 Omicron variant predominance, researchers report in Clinical Infectious Diseases.
Led by researchers from the Infection Control Program and the World Health Organization (WHO) Collaborating Center at Geneva University Hospitals, the retrospective study involved 2,901 symptomatic COVID-19 patients and 868 flu patients at nine hospitals. Cases diagnosed at least 3 days after admission were reported to the national hospital surveillance program from February 2022 to April 2023 (COVID-19) or November 2018 to April 2023 (flu).
"These two diseases represent a substantial burden of respiratory illnesses among hospitalized patients, particularly during the winter months in temperate climates," the study authors noted. "Hospitalized patients are often at increased risk for severe COVID-19 or influenza, particularly older adults, people with weakened immune systems, and those with chronic conditions."
More complications with flu
The absolute number of COVID-19 deaths was three times higher than those from flu, but the case-fatality ratio was comparable for COVID-19 (6.2%) and flu (6.1%); after adjustment, patients had a comparable subdistribution hazard ratio (HR) for 30-day in-hospital death (0.91). Similar percentages were admitted to the ICU (2.4% COVID-19 vs 2.6% flu).
A higher percentage of patients were admitted to an intermediate care unit for COVID-19 (2.9%) than for flu (1.4%), and a greater proportion of flu patients had complications than those with COVID-19 (35.3% vs 29.3%).
The adjusted cause-specific HR for in-hospital death from flu versus COVID-19 was 0.94, and the cause-specific HR for flu-patient hospital release compared with COVID-19 hospital release was 1.04. The weighted subdistribution HR for flu death, compared with those from COVID-19, with hospital discharge as a competing risk was 0.91.
Vaccination shows protective effect
While there was no difference in risk of death by vaccination status (at least one dose vs none), hospital stays were a median of 2 days longer (16 vs 14 days) among unvaccinated COVID-19 patients (16 vs 14 days) and flu patients (16 vs 11). Likewise, COVID-19 patients who had received the last of at least 3 doses within the past year had shorter hospital stays than those who were unvaccinated or had received a vaccine dose more than a year before.
The impact of cumulative vaccine- and infection-derived immune responses likely explains the reduction in hazard for in-hospital mortality for COVID-19 as compared to influenza.
"While the relative hazard may be comparable, it is important to recognize that the absolute number of patients with healthcare-associated COVID-19 (Omicron) was three-fold higher than the number of patients with healthcare-associated influenza," the researchers wrote. "COVID-19 therefore still represents a substantially larger clinical burden for hospitals than influenza in Switzerland."
The finding of shorter hospital stays among the vaccinated confirm vaccines' protective effects, they said.
"The impact of cumulative vaccine- and infection-derived immune responses likely explains the reduction in hazard for in-hospital mortality for COVID-19 as compared to influenza, as well as changes in the inherent virulence of pre-Omicron versus Omicron SARS-CoV-2 variants," the authors wrote. "Although we also cannot exclude that changes in the virulence of influenza strains and pre-existing immunity may also contribute to our observations."
