Clinical manifestations of adult patients requiring influenza-associated hospitalization in the pre-, during, and post-COVID-19 pandemic periods: An internet-based, prospective, multicenter, cohort study in Japan.

Publication date: May 12, 2026

Influenza is a major concern, but it has been affected by the COVID-19 pandemic worldwide. The objective of this study was to clarify the differences in the clinical manifestations of severely ill patients infected with influenza in Japan among the pre-, during, and post-COVID-19 pandemic periods. The clinical data of patients who were severely ill with influenza and required hospitalization were gathered and analyzed between September 2019 and August 2025 (6 influenza seasons) using an internet-based data-capture system, and the pre-(2019 season), during (2020, 2021, and 2022 seasons), and post- (2023 and 2024 seasons) COVID-19 pandemic periods were compared. A total of 490 patients were enrolled and analyzed, with 87 (17. 8%), 21 (4. 29%), and 382 (78. 5%) patients in the pre-, during, and post-COVID-19 pandemic periods, with mortality of 2. 3%, 0%, and 7. 9%, respectively. Age, male/female ratio, underlying diseases, pneumonia rates, oxygen use, isolated bacteria, including Streptococcus pneumoniae, and anti-influenza agents were similar among the three groups. However, intrafamilial transmission was decreased, although the influenza vaccination rate and antibiotic use were decreased in the post-COVID-19 period. Influenza infection might be a greater issue after the COVID-19 pandemic because of the decreased vaccination rate and worse prognosis in the post-COVID-19 period. Although intrafamilial transmission and antibiotic use were decreased in the post-COVID-19 period, continuous nationwide surveillance and promotion of vaccination will be required to inhibit the increase of hospitalized and/or severe influenza patients.

Semantics

Original Article