Community influenza outbreaks and emergency department ambulance diversion

Study objective: Influenza has been linked to emergency department (ED) crowding, yet few studies have examined this association. We describe the relationship between ED crowding and influenza outbreaks. Methods: A retrospective time series analysis was conducted in Toronto from January 1996 to April 1999 (n=170 weeks). We obtained weekly data on laboratory-confirmed influenza and other respiratory virus cases in the community, ED ambulance diversion, and visits to all city EDs (n=20). The main outcome was ambulance diversion, measured as the mean number of hours per week in which EDs were forced to divert all ambulances. Results: A mean of 10,936 ED visits occurred weekly (average age of patients 39.9 years; 51% female patients). EDs diverted ambulances an average of 3.4 hours per week (range 0.3 to 15 hours). Four influenza seasons occurred, lasting between 18 and 30 weeks each, with weekly influenza case counts ranging from 0 to 236. There were fewer than 10 cases per week in 119 of 170 weeks (70%). In time-series models, influenza was independently associated with ED ambulance diversion (P<.0001). For every 100 cases of influenza in the community in a given week, ED ambulance diversion would be expected to increase by 2.5 h/wk at the average ED (95% confidence interval [CI] 1.2 to 3.9 h/wk). During influenza seasons, 24.3% (95% CI 11.3% to 37.2%) of observed weekly ambulance diversion was attributable to influenza. Conclusion: Influenza seasons are associated with increased ED ambulance diversion. The impact is substantial but brief because there is little or no influenza activity most of each year.