title: Comparing estimates of influenza-associated hospitalization and death among adults with congestive heart failure based on how influenza season is defined.
abstract: BACKGROUND
  There is little consensus about how the influenza season should be defined in studies that assess influenza-attributable risk. The objective of this study was to compare estimates of influenza-associated risk in a defined clinical population using four different methods of defining the influenza season.
  
  
  METHODS
  Using the Studies of Left Ventricular Dysfunction (SOLVD) clinical database and national influenza surveillance data from 1986-87 to 1990-91, four definitions were used to assess influenza-associated risk: (a) three-week moving average of positive influenza isolates is at least 5%, (b) three-week moving average of positive influenza isolates is at least 10%, (c) first and last positive influenza isolate are identified, and (d) 5% of total number of positive isolates for the season are obtained. The clinical data were from adults aged 21 to 80 with physician-diagnosed congestive heart failure. All-cause hospitalization and all-cause mortality during the influenza seasons and non-influenza seasons were compared using four definitions of the influenza season. Incidence analyses and Cox regression were used to assess the effect of exposure to influenza season on all-cause hospitalization and death using all four definitions.
  
  
  RESULTS
  There was a higher risk of hospitalization associated with the influenza season, regardless of how the start and stop of the influenza season was defined. The adjusted risk of hospitalization was 8 to 10 percent higher during the influenza season compared to the non-influenza season when the different definitions were used. However, exposure to influenza was not consistently associated with higher risk of death when all definitions were used. When the 5% moving average and first/last positive isolate definitions were used, exposure to influenza was associated with a higher risk of death compared to non-exposure in this clinical population (adjusted hazard ratios [HR], 1.16; 95% confidence interval [CI], 1.04 to 1.29 and adjusted HR, 1.19; 95% CI, 1.06 to 1.33, respectively).
  
  
  CONCLUSIONS
  Estimates of influenza-attributable risk may vary depending on how influenza season is defined and the outcome being assessed.
authors.affiliation: Department of Clinical Epidemiology and Biostatistics, McMaster University, Hamilton, Ontario, Canada. CSandoval@cihi.ca
authors.forename: Carolyn
authors.initials: C
authors.lastname: Sandoval
authors.forename: Stephen D
authors.initials: SD
authors.lastname: Walter
authors.forename: Paul
authors.initials: P
authors.lastname: Krueger
authors.forename: Mark B
authors.initials: MB
authors.lastname: Loeb
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D005260:Female; D006333:Heart Failure / Q000150:complications* / Q000401:mortality; D006760:Hospitalization / Q000706:statistics & numerical data*; D006801:Humans; D007251:Influenza, Human / Q000150:complications / Q000453:epidemiology* / Q000401:mortality; D008297:Male; D008875:Middle Aged; D009656:North America / Q000453:epidemiology; D011159:Population Surveillance; D016016:Proportional Hazards Models; D012307:Risk Factors; D012621:Seasons; D018487:Ventricular Dysfunction, Left