Nosocomial Influenza: Effectiveness of Mandatory Surgical Masks for Prevention, Associated Risk Factors, and Prognostic Value

Background: Nosocomial influenza (NI) represents a potentially preventable adverse event with significant clinical impact, particularly in vulnerable hospitalised populations. During the 2024–2025 influenza season, a stepped programme of mandatory surgical mask use (MSMU) was implemented in response to increasing NI clusters. This study aimed to evaluate the effectiveness of three MSMU-based interventions and to analyse the risk factors associated with NI and its prognostic value for adverse clinical outcomes. Methods: A before–after observational study was conducted in an open cohort of hospitalised patients with PCR‑confirmed influenza A or B in a tertiary‑care hospital. Three MSMU interventions were implemented sequentially at the unit and hospital levels. For each intervention, the burden of NI was assessed using incidence rates per 10⁴ patient‑days and the proportion of nosocomial cases among all influenza cases. Rate ratios (RRt), relative risks (RR), and prevention program effectiveness (PPE) were estimated by comparing for the pre‑intervention (PIP) and intervention (IP) periods. Multivariable logistic regression was used to identify risk factors associated with NI and to evaluate its association with prolonged length of stay (≥11 days), intensive care unit (ICU) admission and all- cause mortality. Results: Among 340 influenza cases, 57 (16.8%) were nosocomial. Across the three interventions, NI incidence rates and proportions were consistently lower during IP than during PIP. PPE estimates based on incidence rates were 71.7%, 71.4%, and 69.9% for the first, second, and third interventions, respectively, based on proportions, PPE estimates were 25.3%, 40.8%, and 78.3%. In multivariable analysis, MSMU was associated with a reduced likelihood of NI (adjusted odds ratio [OR] 0.17), while chronic kidney disease (aOR 1.99) and haematologic disease (aOR 2.28) were independently associated with NI. NI was further associated with prolonged hospitalization (aOR 1.99) and mortality (aOR 4.57). Conclusions: MSMU was associated with a lower burden of NI during periods of high influenza circulation, with increasing effectiveness when applied hospital-wide. NI occurs more frequently in clinically vulnerable patients and was associated with worse outcomes. MSMU may be a useful preventive strategy during seasonal influenza peaks, particularly in high‑risk wards.