Clinical manifestations of hospitalized influenza patients without risk factors: A prospective multicenter cohort study in Japan via internet surveillance

Introduction: Influenza remains a clinically heavy burden worldwide. It is well known that some populations are at high risk of complications from influenza, whereas, even previously healthy people might suffer from severe influenza.The objective of this study was to clarify clinical manifestations of hospitalized patients without risk factors infected with influenza. Methods: The clinical data for patients who were severely ill with influenza, and required hospitalization were gathered and analyzed between November 2014 and August 2020 (6 influenza seasons) using an internetsurveillance system. Among them, the patients who had no risk factors of complications from influenza were extracted.Results: Finally, a total of 91 patients (9.0% of all influenza-related hospitalizations) without risk factors were analyzed. The no risk group was younger than the risk group, though other significant differences of clinical characteristics were not recognized between the groups. Pneumonia was the most common cause of hospitali-zation in the no risk group, and primary influenza viral pneumonia was the most common pneumonia. Antiviral drugs were administered in 96.7% of the no-risk group, and artificial ventilation was performed in 18.7%. In-hospital death was recorded for 3 patients without risk factors.Conclusions: Severe complications of influenza which required hospitalization may occur in a certain degree of patients with no risk factors. Efforts are needed to diagnose and treat influenza appropriately even in previously healthy younger patients. Continuous nationwide surveillance will be required to clarify risk factors for severe influenza even in previously healthy younger patients. (UMIN000015989).