Incidence of medically attended influenza during pandemic and post-pandemic seasons through the Influenza Incidence Surveillance Project, 2009-13

Lancet Respir Med. 2015 Sep;3(9):709-718. doi: 10.1016/S2213-2600(15)00278-7. Epub 2015 Aug 21.


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.

Findings: 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.

Interpretation: 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.

Funding: US Centers for Disease Control and Prevention.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Adolescent
  • Adult
  • Age Distribution
  • Aged
  • Ambulatory Care / statistics & numerical data*
  • Antiviral Agents / therapeutic use
  • Child
  • Child, Preschool
  • Female
  • Hospitalization / statistics & numerical data
  • Humans
  • Incidence
  • Infant
  • Influenza A Virus, H1N1 Subtype
  • Influenza A Virus, H3N2 Subtype
  • Influenza B virus
  • Influenza Vaccines / therapeutic use
  • Influenza, Human / epidemiology*
  • Male
  • Middle Aged
  • Pandemics / statistics & numerical data*
  • Population Surveillance*
  • Seasons
  • United States / epidemiology
  • Vaccination / statistics & numerical data
  • Young Adult


  • Antiviral Agents
  • Influenza Vaccines