Burden and severe outcomes of influenza in comparison to COVID-19 among hospitalised adult patients during the 2024-2025 season: a prospective, multicentre study in Greece.

Aim

To compare the burden and severe outcomes among patients hospitalised for influenza to those hospitalised for coronavirus disease 2019 (COVID-19) in Greece.

Methods

The study was conducted in four hospitals from November 2024 to May 2025. Main outcomes were length of stay (LOS) and severe outcomes (admission to intensive care unit, invasive mechanical ventilation, in-hospital death). The associations between LOS and adverse outcomes with COVID-19 or influenza were estimated with multivariable negative binomial and logistic regression models.

Results

We studied 621 patients [mean age: 75.6 years; 564 (90.8%) with at least one comorbidity], of whom 43 (6.9%) died. In total, 358 patients were admitted for COVID-19 and 263 for influenza. Influenza patients had a mean LOS of 10.7 days compared with 7.5 days among COVID-19 patients (p-value <0.001), while severe outcomes were similar across the two groups. Adjusted models yielded similar findings, with influenza patients having significantly longer LOS [incidence rate ratio (IRR): 1.47; 95% confidence interval (CI): 1.03-2.10] compared with COVID-19 patients, and no difference in the odds of experiencing any severe outcome [adjusted odds ratio (aOR) = 2.10; 95% CI: 0.88-5.04]. Pneumonia was associated with increased odds for any severe outcome (aOR: 7.55; 95% CI: 1.10-52.33) and longer LOS (IRR: 2.09; 95% CI: 1.39-3.15) overall and within diagnostic subgroups. Among COVID-19 patients, those with ≥1 COVID-19 vaccine dose had reduced odds for experiencing any severe outcome (aOR = 0.38; 95% CI: 0.15-0.96) compared with unvaccinated patients.

Conclusions

In 2024-2025, influenza patients had significantly longer LOS than COVID-19 patients, while severe outcomes were not significantly different between the two groups.