title: Incidence of medically attended influenza during pandemic and post-pandemic seasons through the Influenza Incidence Surveillance Project, 2009-13.
abstract: BACKGROUND
  Since the introduction of pandemic influenza A (H1N1) to the USA in 2009, the Influenza Incidence Surveillance Project has monitored the burden of influenza in the outpatient setting through population-based surveillance.
  
  
  METHODS
  From Oct 1, 2009, to July 31, 2013, outpatient clinics representing 13 health jurisdictions in the USA reported counts of influenza-like illness (fever including cough or sore throat) and all patient visits by age. During four years, staff at 104 unique clinics (range 35-64 per year) with a combined median population of 368,559 (IQR 352,595-428,286) attended 35,663 patients with influenza-like illness and collected 13,925 respiratory specimens. Clinical data and a respiratory specimen for influenza testing by RT-PCR were collected from the first ten patients presenting with influenza-like illness each week. We calculated the incidence of visits for influenza-like illness using the size of the patient population, and the incidence attributable to influenza was extrapolated from the proportion of patients with positive tests each week.
  
  
  RESULTS
  The site-median peak percentage of specimens positive for influenza ranged from 58.3% to 77.8%. Children aged 2 to 17 years had the highest incidence of influenza-associated visits (range 4.2-28.0 per 1000 people by year), and adults older than 65 years had the lowest (range 0.5-3.5 per 1000 population). Influenza A H3N2, pandemic H1N1, and influenza B equally co-circulated in the first post-pandemic season, whereas H3N2 predominated for the next two seasons. Of patients for whom data was available, influenza vaccination was reported in 3289 (28.7%) of 11,459 patients with influenza-like illness, and antivirals were prescribed to 1644 (13.8%) of 11,953 patients.
  
  
  CONCLUSIONS
  Influenza incidence varied with age groups and by season after the pandemic of 2009 influenza A H1N1. High levels of influenza virus circulation, especially in young children, emphasise the need for additional efforts to increase the uptake of influenza vaccines and antivirals.
  
  
  BACKGROUND
  US Centers for Disease Control and Prevention.
authors: Influenza Division, Centers for Disease Control and Prevention, Atlanta, GA, USA.
authors: Ashley
authors: A
authors: Fowlkes
authors: Influenza Division, Centers for Disease Control and Prevention, Atlanta, GA, USA.
authors: Andrea
authors: A
authors: Steffens
authors: Department of Family Medicine, University of Wisconsin School of Medicine and Public Health, Madison, WI, USA.
authors: Jon
authors: J
authors: Temte
authors: Division of Prevention and Primary Care, New York City Department of Health and Mental Hygiene, Gotham Center, Long Island City, NY, USA.
authors: Steve Di
authors: SD
authors: Lonardo
authors: Communicable Disease Service Infectious and Zoonotic Disease, New Jersey Department of Health, Trenton, NJ, USA.
authors: Lisa
authors: L
authors: McHugh
authors: Minnesota Department of Health, St Paul, MN, USA.
authors: Karen
authors: K
authors: Martin
authors: Bureau of Epidemiology, Florida Department of Health, Tallahassee, FL, USA.
authors: Heather
authors: H
authors: Rubino
authors: Division of Disease Control, North Dakota Department of Health, Bismarck, ND, USA.
authors: Michelle
authors: M
authors: Feist
authors: Public Health Preparedness and Epidemiology Program, Texas Department of Health, Temple, TX, USA.
authors: Carol
authors: C
authors: Davis
authors: Acute Communicable Disease Control Program, Los Angeles County Department of Public Health, Los Angeles, CA, USA.
authors: Christine
authors: C
authors: Selzer
authors: Division of Disease Control, Philadelphia Department of Public Health, Philadelphia, PA, USA.
authors: Jose
authors: J
authors: Lojo
authors: Center for Acute Disease Epidemiology, Iowa Department of Public Health, Des Moines, IA, USA.
authors: Oluwakemi
authors: O
authors: Oni
authors: Division of Surveillance and Investigation, Virginia Department of Health, Richmond, VA, USA.
authors: Katie
authors: K
authors: Kurkjian
authors: Acute and Communicable Disease Program, Oregon Public Health Division, Portland, OR, USA.
authors: Ann
authors: A
authors: Thomas
authors: Division of Disease Control and Prevention, Utah Department of Health, Salt Lake City, UT, USA.
authors: Rachelle
authors: R
authors: Boulton
authors: Council of State and Territorial Epidemiologists, Atlanta, GA, USA. Electronic address: afowlkes@cdc.gov.
authors: Nicole
authors: N
authors: Bryan
authors: Minnesota Department of Health, St Paul, MN, USA.
authors: Ruth
authors: R
authors: Lynfield
authors: Influenza Division, Centers for Disease Control and Prevention, Atlanta, GA, USA.
authors: Matthew
authors: M
authors: Biggerstaff
authors: Influenza Division, Centers for Disease Control and Prevention, Atlanta, GA, USA.
authors: Lyn
authors: L
authors: Finelli
mesh_terms: D000293:Adolescent; D000328:Adult; D017677:Age Distribution; D000368:Aged; D000553:Ambulatory Care / Q000706:statistics & numerical data*; D000998:Antiviral Agents / Q000627:therapeutic use; D002648:Child; D002675:Child, Preschool; D005260:Female; D006760:Hospitalization / Q000706:statistics & numerical data; D006801:Humans; D015994:Incidence; D007223:Infant; D053118:Influenza A Virus, H1N1 Subtype; D053122:Influenza A Virus, H3N2 Subtype; D009981:Influenza B virus; D007252:Influenza Vaccines / Q000627:therapeutic use; D007251:Influenza, Human / Q000453:epidemiology*; D008297:Male; D008875:Middle Aged; D058873:Pandemics / Q000706:statistics & numerical data*; D011159:Population Surveillance; D012621:Seasons; D014481:United States / Q000453:epidemiology; D014611:Vaccination / Q000706:statistics & numerical data; D055815:Young Adult
chemical_list: D000998:Antiviral Agents; D007252:Influenza Vaccines