Epidemiologic patterns of influenza outbreaks in institutional settings

Congregate environments facilitate spread of influenza viruses and could result in serious outcomes among residents.

We reviewed the epidemiology and identified factors that affect outbreak outcomes, using descriptive statistics and generalized estimating equations (GEE) regression. Outbreak duration, attack rate, hospitalization rate, case fatality rate, and the proportion of laboratory tests performed among infected residents were calculated by viral etiology, time to notification, and facility type and size.

Between January 2011 and May 2015, the median attack rate for the 41 influenza outbreaks was highest among children in kindergarten (34%) and lowest among patients, who acquired influenza during hospitalization (17%). The median number of cases in each outbreak was 37 (range: 6-158) and a median of 199 individuals (range: 25-574) was exposed to influenza. The outbreaks lasted 2-49 days (median: 13 days). The average case fatality rate among residents for outbreaks attributed to influenza A or B was 3.1/1000 and for outbreaks attributed to influenza B 6.7/1000. Median time to notification was 10 days after the index case. In multivariate analysis, influenza outbreaks reported sooner after onset in the primary case ended sooner (P = 0.027) and had lower case fatality rates (P < 0.001). Residing in larger facilities was associated with lower proportion of laboratory tests performed (P = 0.024).

Understanding whether earlier notification is a measure of a better functioning and therefore safer facility is a potentially important measure of the quality of care.