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

Influenza Other Respir Viruses. 2013 Sep;7(5):799-808. doi: 10.1111/irv.12011. Epub 2012 Nov 5.


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 <65 years was approximately six times higher during the 2009 H1N1 pandemic, whereas for persons 75 years 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.

Keywords: Case ascertainment; data analysis; empirical research; hospital admissions; influenza; seasonal and pandemic; statistical models.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adolescent
  • Adult
  • Aged
  • Canada / epidemiology
  • Child
  • Child, Preschool
  • Female
  • Hospitalization
  • Humans
  • Influenza A Virus, H1N1 Subtype / genetics
  • Influenza A Virus, H1N1 Subtype / isolation & purification*
  • Influenza, Human / epidemiology*
  • Influenza, Human / virology
  • Male
  • Middle Aged
  • Pandemics
  • Patient Admission
  • Seasons
  • Young Adult