Hospital-acquired influenza: a synthesis using the Outbreak Reports and Intervention Studies of Nosocomial Infection (ORION) statement.

Nosocomial influenza outbreaks occur in almost all types of hospital wards, and their consequences for patients and hospitals in terms of morbidity, mortality and costs are considerable. The source of infection is often unknown, since any patient, healthcare worker (HCW) or visitor is capable of transmitting it to susceptible persons within hospitals. Nosocomial influenza outbreak investigations should help to identify the source of infection, prevent additional cases, and increase our knowledge of disease control to face future outbreaks. However, such outbreaks are probably underdetected and underreported, making routes of transmission difficult to track and describe with precision. In addition, the absence of standardised information in the literature limits comparison between studies and better understanding of disease dynamics. In this study, reports of nosocomial influenza outbreaks are synthesised according to the ORION guidelines to highlight existing knowledge in relation to the detection of influenza cases, evidence of transmission between patients and HCWs and measures of disease incidence. Although a body of evidence has confirmed that influenza spreads within hospitals, we should improve clinical and virological confirmation and initiate active surveillance and quantitative studies to determine incidence rates in order to assess the risk to patients.