title: Estimating influenza incidence and rates of influenza-like illness in the outpatient setting.
abstract: BACKGROUND
  Estimating influenza incidence in outpatient settings is challenging. We used outpatient healthcare practice populations as a proxy to estimate community incidence of influenza-like illness (ILI) and laboratory-confirmed influenza-associated ILI.
  
  
  METHODS
  From October 2009 to July 2010, 38 outpatient practices in seven jurisdictions conducted surveillance for ILI (fever with cough or sore throat for patients ≥ 2 years; fever with ≥ 1 respiratory symptom for patients <2 years). From a sample of patients with ILI, respiratory specimens were tested for influenza.
  
  
  RESULTS
  During the week of peak influenza activity (October 24, 2009), 13% of outpatient visits were for ILI and influenza was detected in 72% of specimens. For the 10-month surveillance period, ILI and influenza-associated ILI incidence were 20.0 (95% CI: 19.7, 20.4) and 8.7/1000 (95% CI: 8.2, 9.2) persons, respectively. Influenza-associated ILI incidence was highest among children aged 2-17 years. Observed trends were highly correlated with national ILI and virologic surveillance.
  
  
  CONCLUSIONS
  This is the first multistate surveillance system demonstrating the feasibility of using outpatient practices to estimate the incidence of medically attended influenza at the community level. Surveillance demonstrated the substantial burden of pandemic influenza in outpatient settings and especially in children aged 2-17 years. Observed trends were consistent with established syndromic and virologic systems.
authors.affiliation: Influenza Division, National Center for Immunization and Respiratory Diseases, Centers for Disease Control and Prevention, Atlanta, GA 30333, USA. afowlkes@cdc.gov
authors.forename: Ashley
authors.initials: A
authors.lastname: Fowlkes
authors.forename: Sharoda
authors.initials: S
authors.lastname: Dasgupta
authors.forename: Edward
authors.initials: E
authors.lastname: Chao
authors.forename: Jennifer
authors.initials: J
authors.lastname: Lemmings
authors.forename: Kate
authors.initials: K
authors.lastname: Goodin
authors.forename: Meghan
authors.initials: M
authors.lastname: Harris
authors.forename: Karen
authors.initials: K
authors.lastname: Martin
authors.forename: Michelle
authors.initials: M
authors.lastname: Feist
authors.forename: Winfred
authors.initials: W
authors.lastname: Wu
authors.forename: Rachelle
authors.initials: R
authors.lastname: Boulton
authors.forename: Jonathan
authors.initials: J
authors.lastname: Temte
authors.forename: Lynnette
authors.initials: L
authors.lastname: Brammer
authors.forename: Lyn
authors.initials: L
authors.lastname: Finelli
mesh_terms: D000293:Adolescent; D000328:Adult; D017677:Age Distribution; D000368:Aged; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D015994:Incidence; D007223:Infant; D053118:Influenza A Virus, H1N1 Subtype / Q000235:genetics / Q000302:isolation & purification*; D007251:Influenza, Human / Q000175:diagnosis* / Q000453:epidemiology* / Q000821:virology; D008297:Male; D008875:Middle Aged; D010045:Outpatients / Q000706:statistics & numerical data*; D011159:Population Surveillance; D014481:United States / Q000453:epidemiology; D055815:Young Adult