2.1 Patient recruitment
For this study, patients for whom influenza virus nucleic acid testing was performed in the microbiology labs of five tertiary hospitals in China (Supplementary Material 1) from January 1st, 2012 – December 31st, 2019 were screened for eligibility. Those patients with confirmed Flu-p were enrolled in this study. Patients were excluded if they: (i) were not classified as having community-onset pneumonia (hospitalized within the last 28 days, with pneumonia onset ≥ 48 h after admission [10]), as the inclusion of nosocomial pneumonia cases had the potential to complicate result interpretations; (ii) were < 18 years old; (iii) were immunocompromised, given that this had the potential to adversely impact influenza clinical outcomes [11]; or (ⅳ) had been intubated prior to admission.
The study design was approved by the Ethics Committee of Beijing Jishuitan Hospital (No.201911-15). Given the retrospective nature of the study, the Ethics Committee determined that an informed consent was not necessary.