title: Influenza-associated hospitalizations in the United States.
abstract: BACKGROUND
  Respiratory viral infections are responsible for a large number of hospitalizations in the United States each year.
  
  
  OBJECTIVE
  To estimate annual influenza-associated hospitalizations in the United States by hospital discharge category, discharge type, and age group.
  
  
  METHODS
  National Hospital Discharge Survey (NHDS) data and World Health Organization Collaborating Laboratories influenza surveillance data were used to estimate annual average numbers of hospitalizations associated with the circulation of influenza viruses from the 1979-1980 through the 2000-2001 seasons in the United States using age-specific Poisson regression models.
  
  
  METHODS
  We estimated influenza-associated hospitalizations for primary and any listed pneumonia and influenza and respiratory and circulatory hospitalizations.
  
  
  RESULTS
  Annual averages of 94,735 (range, 18,908-193,561) primary and 133,900 (range, 30,757-271,529) any listed pneumonia and influenza hospitalizations were associated with influenza virus infections. Annual averages of 226,54 (range, 54,523-430,960) primary and 294,128 (range, 86,494-544,909) any listed respiratory and circulatory hospitalizations were associated with influenza virus infections. Persons 85 years or older had the highest rates of influenza-associated primary respiratory and circulatory hospitalizations (1194.9 per 100,000 persons). Children younger than 5 years (107.9 primary respiratory and circulatory hospitalizations per 100,000 persons) had rates similar to persons aged 50 through 64 years. Estimated rates of influenza-associated hospitalizations were highest during seasons in which A(H3N2) viruses predominated, followed by B and A(H1N1) seasons. After adjusting for the length of each influenza season, influenza-associated primary pneumonia and influenza hospitalizations increased over time among the elderly. There were no significant increases in influenza-associated primary respiratory and circulatory hospitalizations after adjusting for the length of the influenza season.
  
  
  CONCLUSIONS
  Significant numbers of influenza-associated hospitalizations in the United States occur among the elderly, and the numbers of these hospitalizations have increased substantially over the last 2 decades due in part to the aging of the population. Children younger than 5 years had rates of influenza-associated hospitalizations similar to those among individuals aged 50 through 64 years. These findings highlight the need for improved influenza prevention efforts for both young and older US residents.
authors.affiliation: Epidemiology and Surveillance Division, National Immunization Program, Centers for Disease Control and Prevention, Atlanta, Ga 30333, USA. wct2@cdc.gov
authors.forename: William W
authors.initials: WW
authors.lastname: Thompson
authors.forename: David K
authors.initials: DK
authors.lastname: Shay
authors.forename: Eric
authors.initials: E
authors.lastname: Weintraub
authors.forename: Lynnette
authors.initials: L
authors.lastname: Brammer
authors.forename: Carolyn B
authors.initials: CB
authors.lastname: Bridges
authors.forename: Nancy J
authors.initials: NJ
authors.lastname: Cox
authors.forename: Keiji
authors.initials: K
authors.lastname: Fukuda
mesh_terms: D000293:Adolescent; D000328:Adult; D017677:Age Distribution; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D006760:Hospitalization / Q000706:statistics & numerical data*; D006801:Humans; D009980:Influenza A virus; D009981:Influenza B virus; D007251:Influenza, Human / Q000150:complications / Q000453:epidemiology* / Q000821:virology; D038801:International Classification of Diseases; D007902:Length of Stay; D008875:Middle Aged; D009017:Morbidity; D010351:Patient Discharge; D011014:Pneumonia / Q000453:epidemiology; D016012:Poisson Distribution; D012621:Seasons; D014481:United States / Q000453:epidemiology