Statistical estimates of respiratory admissions attributable to seasonal and pandemic influenza for Canada

Abstract

Background: The number of admissions to hospital for which influenza is laboratory confirmed is considered to be a substantial underestimate of the true number of admissions due to an influenza infection. During the 2009 pandemic, testing for influenza in hospitalized patients was a priority, but the ascertainment rate remains uncertain. Methods The discharge abstracts of persons admitted with any respiratory condition were extracted from the Canadian Discharge Abstract Database, for April 2003-March 2010. Stratified, weekly admissions were modeled as a function of viral activity, seasonality, and trend using Poisson regression models. Results An estimated 1 out of every 6·4 admissions attributable to seasonal influenza (2003-April 2009) were coded to J10 (influenza virus identified). During the 2009 pandemic (May-March 2010), the influenza virus was identified in 1 of 1·6 admissions (95% CI, 1·5-1·7) attributed to the pandemic strain. Compared with previous H1N1 seasons (2007/08, 2008/09), the influenza-attributed hospitalization rate for persons <65years was approximately six times higher during the 2009 H1N1 pandemic, whereas for persons 75years or older, the pandemic rate was approximately fivefold lower. Conclusions Case ascertainment was much improved during the pandemic period, with under ascertainment of admissions due to H1N1/2009 limited primarily to patients with a diagnosis of pneumonia. © 2012 John Wiley & Sons Ltd and Her Majesty the Queen in Right of Canada 2012. Reproduced with the permission of the Minister of the Public Health Agency of Canada.