Impact of seasonal and pandemic influenza on emergency department visits, 2003-2010, Ontario, Canada

Abstract

Objectives: Weekly influenza-like illness (ILI) consultation rates are an integral part of influenza surveillance. However, in most health care settings, only a small proportion of true influenza cases are clinically diagnosed as influenza or ILI. The primary objective of this study was to estimate the number and rate of visits to the emergency department (ED) that are attributable to seasonal and pandemic influenza and to describe the effect of influenza on the ED by age, diagnostic categories, and visit disposition. A secondary objective was to assess the weekly "real-time" time series of ILI ED visits as an indicator of the full burden due to influenza.

Methods: The authors performed an ecologic analysis of ED records extracted from the National Ambulatory Care Reporting System (NARCS) database for the province of Ontario, Canada, from September 2003 to March 2010 and stratified by diagnostic characteristics (International Classification of Diseases, 10th Revision [ICD-10]), age, and visit disposition. A regression model was used to estimate the seasonal baseline. The weekly number of influenza-attributable ED visits was calculated as the difference between the weekly number of visits predicted by the statistical model and the estimated baseline.

Results: The estimated rate of ED visits attributable to influenza was elevated during the H1N1/2009 pandemic period at 1,000 per 100,000 (95% confidence interval [CI] = 920 to 1,100) population compared to an average annual rate of 500 per 100,000 (95% CI = 450 to 550) for seasonal influenza. ILI or influenza was clinically diagnosed in one of 2.6 (38%) and one of 14 (7%) of these visits, respectively. While the ILI or clinical influenza diagnosis was the diagnosis most specific to influenza, only 87% and 58% of the clinically diagnosed ILI or influenza visits for pandemic and seasonal influenza, respectively, were likely directly due to an influenza infection. Rates for ILI ED visits were highest for younger age groups, while the likelihood of admission to hospital was highest in older persons. During periods of seasonal influenza activity, there was a significant increase in the number of persons who registered with nonrespiratory complaints, but left without being seen. This effect was more pronounced during the 2009 pandemic. The ratio of influenza-attributed respiratory visits to influenza-attributed ILI visits varied from 2.4:1 for the fall H1N1/2009 wave to 9:1 for the 2003/04 influenza A(H3N2) season and 28:1 for the 2007/08 H1N1 season.

Conclusions: Influenza appears to have had a much larger effect on ED visits than was captured by clinical diagnoses of influenza or ILI. Throughout the study period, ILI ED visits were strongly associated with excess respiratory complaints. However, the relationship between ILI ED visits and the estimated effect of influenza on ED visits was not consistent enough from year to year to predict the effect of influenza on the ED or downstream in-hospital resource requirements.

© 2013 by the Society for Academic Emergency Medicine.

Figures

Figure 1

Weekly ED visits, Ontario, Canada, 2004/05–2009/10, NACRS Database, CIHI, showing model fit, estimated baseline, and excess visits attributed to influenza. The estimated baseline curve (thick red line) accounts for seasonality and secular trends inherent in (A) total ED visits, (B) respiratory visits, and (C) ILI visits, but in the absence of influenza activity. Model-predicted values (open diamonds) correspond closely to the actual number of visits (thin line). The excess number of visits attributed to influenza is the difference between model predicted and baseline. As total ED visits are relatively lower during winter months compared to summer months, the excess due to seasonal influenza typically did not correspond to peak visits, unless peak influenza activity aligned with weeks 52 and 1 (Christmas/New Year's period). The 2009 fall pandemic wave was a significant exception, with ED visits increasing to 1.3 times the usual peak levels and influenza accounting for 30% of weekly ED visits at the peak (A). H1N1/2009 accounted for only 3% of total ED visits for the pandemic period (May 2009–March 2010). The increase in baseline ILI visits once the pandemic was announced is seen in the log-scale insert (C). Seasonal differences between respiratory visits attributed to influenza and influenza-attributed ILI visits are highlighted in (D) and the weekly differences between excess respiratory visits (actual – baseline) and respiratory visits attributed to influenza (model predicted – baseline) are shown in (E). CIHI = Canadian Institute of Health Information; ILI = influenza-like illness; NACRS = National Ambulatory Care Reporting System.