Risk of nosocomial respiratory syncytial virus versus influenza among adult patients in acute care hospitals

Background: While nosocomial influenza is common, the risk of transmission in acute care remains unclear in the absence of systematic surveillance. Even less is known about the risk of nosocomial respiratory syncytial virus (RSV) among immunocompetent adults. Aim: To compare the proportions and incidences of nosocomial cases caused by these two similar respiratory infections among hospitalized adults. Methods: A retrospective study was conducted at two tertiary care hospitals in Southern Finland. Data on all hospitalized adult patients with a positive RSV or influenza test during 2016-22 were used to detect all nosocomial and community-acquired RSV and influenza cases. The proportion of nosocomial cases of all hospitalized cases was calculated. The incidences of nosocomial cases per 1000 bed-days were calculated by season and ward type for the five seasons before the COVID-19 pandemic. Results: Nosocomial RSV and influenza occurred in 2.8% and 8.1% of all hospitalized adult patients with a laboratory-confirmed infection. Over five seasons, 2016-20, the total incidences of nosocomial RSV and influenza cases per 1000 bed-days were 0.027 (95% confidence interval: 0.013, 0.050) and 0.32 (0.27, 0.39). Nosocomial RSV infections were especially poorly recorded with a virus-specific ICD-10 diagnosis code listed for only 16.7% of RSV and 59.8% of nosocomial influenza patients. Conclusion: Despite preventive measures, the incidence of nosocomial influenza was more than tenfold, and the proportion of nosocomial cases was almost threefold compared with RSV among hospitalized adults in acute care. Prevention and surveillance of both nosocomial influenza and RSV should be improved also among immunocompetent adult patients. (c) 2025 Published by Elsevier Ltd on behalf of The Healthcare Infection Society. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).