Unraveling Poland's unprecedented influenza surge in early 2025: increased viral severity or post-pandemic vulnerability?

Background: At the end of 2024, Polish infectious disease specialists observed a sharp increase in influenza hospitalizations, raising concerns about potential underlying causes. This study aimed to analyze differences in patient characteristics, disease progression, and outcomes among individuals hospitalized for influenza in January 2025 compared to January 2024.

Methods: We conducted an exploratory retrospective comparative study across leading infectious disease units in Poland, evaluating demographic data, clinical presentations, treatment regimens, and outcomes of hospitalized influenza patients from both periods. Key variables included influenza type, age, sex distribution, symptom profile, oxygen saturation, inflammatory markers, presence of co-infections, and type of treatment.

Results: Hospitalizations surged by 630% in January 2025, with in-hospital mortality nearly quadrupling to 10.7%. Despite this, patients from both periods had comparable demographic and clinical admission profiles. Most were treated with oseltamivir (though its use was below 90%), and most required antibiotics for bacterial co-infections. Unsurprisingly, the vast majority of hospitalized patients (97%) and non-survivors (93%) in 2025 were unvaccinated. Among non-survivors in 2025, all were infected with influenza A, were older, had higher rates of chronic peripheral circulatory failure, chronic kidney disease, and immunodeficiency, and exhibited more severe inflammatory responses, lower oxygen saturation, and a higher prevalence of dyspnea.

Conclusion: The observed surge likely reflects a post-pandemic phenomenon in a vulnerable, aging, comorbid, and largely unvaccinated population. The findings highlight the urgent need for enhanced influenza vaccination strategies in high-risk groups in Poland, as well as the importance of maintaining continuous antiviral availability throughout the epidemic season. Further research encompassing full-season comparisons and incorporating virological, immunological, and health system factors are warranted to better understand the drivers of such surges and guide future preparedness efforts.