title: 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 <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.
authors.affiliation: Infectious Disease Prevention and Control Branch, Public Health Agency of Canada, Ottawa, ON, Canada. Dena.Schanzer@phac-aspc.gc.ca
authors.forename: Dena L
authors.initials: DL
authors.lastname: Schanzer
authors.forename: Allison
authors.initials: A
authors.lastname: McGeer
authors.forename: Kathleen
authors.initials: K
authors.lastname: Morris
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D002170:Canada / Q000453:epidemiology; D002648:Child; D002675:Child, Preschool; D005260:Female; D006760:Hospitalization; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000235:genetics / Q000302:isolation & purification*; D007251:Influenza, Human / Q000453:epidemiology* / Q000821:virology; D008297:Male; D008875:Middle Aged; D058873:Pandemics; D010343:Patient Admission; D012621:Seasons; D055815:Young Adult
keywords: Case ascertainment; data analysis; empirical research; hospital admissions; influenza; seasonal and pandemic; statistical models