title: Differences in the epidemiological characteristics and clinical outcomes of pandemic (H1N1) 2009 influenza, compared with seasonal influenza.
abstract: BACKGROUND
  There are limited data comparing the clinical presentations, comorbidities, and outcomes of patients with infections due to seasonal influenza with patients with infections due to pandemic (H1N1) 2009 influenza.
  
  
  OBJECTIVE
  To compare the epidemiological characteristics and outcomes of pandemic (H1N1) 2009 influenza with those of seasonal influenza.
  
  
  METHODS
  A cross-sectional study was conducted among patients who received diagnoses during emergency department and inpatient encounters at 2 affiliated academic medical centers in Philadelphia, Pennsylvania. Cases of seasonal influenza during the period November 1, 2005, through June 1, 2008, and cases of pandemic influenza during the period from May 1, 2009, through August 7, 2009, were identified retrospectively.
  
  
  RESULTS
  Forty-nine cases of pandemic influenza and 503 cases of seasonal influenza were identified. Patients with pandemic H1N1 were younger (median age, 29 years) than patients with seasonal influenza (median age, 59 years) (P<.001). More patients with pandemic H1N1 (35 [71%] of 49) were African American, compared with patients with seasonal influenza (267 [53%] of 503; P=.02). Several symptoms were more common among patients with pandemic influenza infections than among patients with seasonal influenza infections: cough (98% vs 83%; P=.007), myalgias (71% vs 46%; P=.001), and pleuritic chest pain (45% vs 15%; P<.001). Pregnancy was the only comorbidity that occurred significantly more often in the pandemic influenza group than in the seasonal influenza group (16% vs 1%; P<.001). There were no significant differences in frequencies of deaths of hospitalized patients, intensive care unit admission, or length of hospitalization between groups.
  
  
  CONCLUSIONS
  Other than pregnancy, there were few clinically important differences between infections due to seasonal influenza and those due to pandemic influenza. The greater rate of lower respiratory tract symptoms in pandemic cases might serve to differentiate pandemic influenza from seasonal influenza.
authors.affiliation: Division of Infectious Diseases, Department of Medicine, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania, USA. shileyk@uphs.upenn.edu
authors.forename: Kevin T
authors.initials: KT
authors.lastname: Shiley
authors.forename: Gregory
authors.initials: G
authors.lastname: Nadolski
authors.forename: Timothy
authors.initials: T
authors.lastname: Mickus
authors.forename: Neil O
authors.initials: NO
authors.lastname: Fishman
authors.forename: Ebbing
authors.initials: E
authors.lastname: Lautenbach
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D015897:Comorbidity; D003430:Cross-Sectional Studies; D004196:Disease Outbreaks; D005260:Female; D006760:Hospitalization; D006785:Hospitals, University; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000302:isolation & purification / Q000472:pathogenicity; D009980:Influenza A virus / Q000302:isolation & purification / Q000472:pathogenicity; D009981:Influenza B virus / Q000302:isolation & purification / Q000472:pathogenicity; D007251:Influenza, Human / Q000453:epidemiology / Q000503:physiopathology / Q000821:virology; D008297:Male; D008875:Middle Aged; D015143:Philadelphia / Q000453:epidemiology; D011247:Pregnancy; D011251:Pregnancy Complications, Infectious / Q000453:epidemiology / Q000503:physiopathology / Q000821:virology; D012621:Seasons; D055815:Young Adult