title: Influenza pneumonia surveillance among hospitalized adults may underestimate the burden of severe influenza disease.
abstract: BACKGROUND
  Studies seeking to estimate the burden of influenza among hospitalized adults often use case definitions that require presence of pneumonia. The goal of this study was to assess the extent to which restricting influenza testing to adults hospitalized with pneumonia could underestimate the total burden of hospitalized influenza disease.
  
  
  METHODS
  We conducted a modelling study using the complete State Inpatient Databases from Arizona, California, and Washington and regional influenza surveillance data acquired from CDC from January 2003 through March 2009. The exposures of interest were positive laboratory tests for influenza A (H1N1), influenza A (H3N2), and influenza B from two contiguous US Federal Regions encompassing the study area. We identified the two outcomes of interest by ICD-9-CM code: respiratory and circulatory hospitalizations, as well as critical illness hospitalizations (acute respiratory failure, severe sepsis, and in-hospital death). We linked the hospitalization datasets with the virus surveillance datasets by geographic region and month of hospitalization. We used negative binomial regression models to estimate the number of influenza-associated events for the outcomes of interest. We sub-categorized these events to include all outcomes with or without pneumonia diagnosis codes.
  
  
  RESULTS
  We estimated that there were 80,834 (95% CI 29,214-174,033) influenza-associated respiratory and circulatory hospitalizations and 26,760 (95% CI 14,541-47,464) influenza-associated critical illness hospitalizations. When a pneumonia diagnosis was excluded, the estimated number of influenza-associated respiratory and circulatory hospitalizations was 24,816 (95% CI 6,342-92,624). The estimated number of influenza-associated critical illness hospitalizations was 8,213 (95% CI 3,764-20,799). Around 30% of both influenza-associated respiratory and circulatory hospitalizations, as well as influenza-associated critical illness hospitalizations did not have pneumonia diagnosis codes.
  
  
  CONCLUSIONS
  Surveillance studies which only consider hospitalizations that include a diagnosis of pneumonia may underestimate the total burden of influenza hospitalizations.
authors.affiliation: Department of Medicine, University of Washington, Seattle, Washington, United States of America; Department of Global Health, University of Washington, Seattle, Washington, United States of America; Vaccine Access and Delivery Global Program, PATH, Seattle, Washington, United States of America.
authors.forename: Justin R
authors.initials: JR
authors.lastname: Ortiz
authors.affiliation: Department of Medicine, University of Washington, Seattle, Washington, United States of America; Department of Global Health, University of Washington, Seattle, Washington, United States of America; Vaccine Access and Delivery Global Program, PATH, Seattle, Washington, United States of America.
authors.forename: Kathleen M
authors.initials: KM
authors.lastname: Neuzil
authors.affiliation: Department of Medicine, University of Michigan, Ann Arbor, Michigan, United States of America.
authors.forename: Colin R
authors.initials: CR
authors.lastname: Cooke
authors.affiliation: The Mountain-Whisper-Light Statistics, Seattle, Washington, United States of America.
authors.forename: Moni B
authors.initials: MB
authors.lastname: Neradilek
authors.affiliation: Department of Medicine, University of Washington, Seattle, Washington, United States of America.
authors.forename: Christopher H
authors.initials: CH
authors.lastname: Goss
authors.affiliation: Influenza Division, Centers for Disease Control and Prevention, Atlanta, Georgia, United States of America.
authors.forename: David K
authors.initials: DK
authors.lastname: Shay
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D006760:Hospitalization; D006801:Humans; D007251:Influenza, Human / Q000150:complications* / Q000453:epidemiology; D007297:Inpatients; D008875:Middle Aged; D018410:Pneumonia, Bacterial / Q000150:complications* / Q000453:epidemiology; D011159:Population Surveillance; D012720:Severity of Illness Index; D014481:United States / Q000453:epidemiology; D055815:Young Adult