Clinical Characteristics Associated with Mortality between Surviving and Non-surviving Adult Patients with Severe Influenza Requiring Hospitalization: An Internet-based, Prospective, Multicenter, Cohort Study in Japan.

Introduction　Influenza remains a huge issue, but its prognosis and clinical characteristics are still unclear. This study aimed to clarify the clinical differences between adults hospitalized with influenza between survivors and nonsurvivors.Methods　Clinical data of patients who were severely ill with influenza and required hospitalization were gathered and analyzed between September 2019 and August 2025 (six influenza seasons) using an internet surveillance system; patients who did not survive were excluded.Results　Of the 490 patients with severe influenza, 32 (6.53%) did not survive and were older than the surviving patients. In the non-surviving patients, very few or none had been vaccinated against influenza or pneumococcal infections. The non-surviving patients had more pure viral pneumonia and more received mechanical ventilation. The male-to-female ratio, underlying diseases, virus type, and use of antibiotics were similar between survivors and non-survivors.Conclusions　Non-surviving patients were older and developed more severe respiratory failure due to pure viral pneumonia than surviving patients. Very low vaccination rates for influenza and pneumococcus might be suggested in hospitalized adults with severe influenza.