An outbreak report of Influenza A(H3) among residents of a large nursing home in northern Italy: attack rates, clinical features and outcomes

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Abstract Background Nursing homes are high-risk settings for Influenza outbreaks, and the 2025–2026 Influenza season has been characterised by an earlier-than-usual epidemic onset globally with the emergence of a novel Influenza A(H3N2) subclade K strain including mutations that were not present in the strains of the 2025–2026 seasonal vaccine. Detailed outbreak investigations are essential to inform Infection Prevention and Control (IPC) strategies in these high-risk yet often neglected settings. Methods This is an outbreak report originating from data collected within an ongoing prospective surveillance in a 375-bed publicly funded nursing home in northern Italy. Surveillance for acute respiratory infection (ARI) was applied to all residents enrolled in the study (November-December 2025). Nasopharyngeal swabs were collected from all ARI cases and analysed for Influenza A(H3). Cases were defined as residents with ARI and laboratory-confirmed Influenza A(H3). Attack rates (ARs), case-hospitalisation rate (CHR), and case-fatality rate (CFR) were calculated with 95% confidence intervals. Findings are here reported according to the Outbreak Reports and Intervention Studies of Nosocomial infection (ORION) guidelines. Results Of 217 involved residents (median age 88 years, 83% Influenza vaccinated), 50 (AR = 23%; 95% CI 17.6–29.2) had laboratory-confirmed Influenza A(H3). Fever (89.8%) and cough (81.6%) were the predominant symptoms. Median symptom duration was 3 days (range 1–14 days). The CHR was 8% (4/50; 95% CI 2.2–19.2) and the CFR was 0.0% (0/50; 95% CI 0.0-7.1) among confirmed cases. Conclusions This outbreak of Influenza A(H3) documented an overall attack rate slightly below the published median for nursing home outbreaks, with a low CHR and no confirmed deaths among laboratory-confirmed cases. The outbreak commenced in week 48, earlier than the historical Italian epidemic onset, underscoring the need for proactive surveillance, sustained attention to Influenza vaccination and IPC measures in long-term care facilities even ahead of the traditional winter peak.

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