Detection of hospital outbreaks of influenza-like illness based on excess of incidence rates compared to the community

The risk of nosocomial influenza-like illness (noso-ILI) compared with that of community-acquired ILI was calculated during 3 influenza seasons (2004-2007) at a 1100-bed university hospital with a total of 21,519 hospitalized patients. Outbreaks of noso-ILI occurred in each season, although a protective effect against noso-ILI was also identified for other wards.