title: The first, second and third wave of pandemic influenza A (H1N1)pdm09 in North Denmark Region 2009-2011: a population-based study of hospitalizations.
abstract: OBJECTIVE
  Denmark experienced three waves of the new pandemic influenza A (H1N1)pdm09 from July 2009 to February 2011. The aim of the study was to describe the epidemiology and clinical characteristics of hospitalized patients in a defined population of North Denmark Region with a mixed urban and rural community of 579,000 inhabitants.
  
  
  METHODS
  Review of medical records of all hospitalized patients with confirmed influenza A from July 2009 to February 2011.
  
  
  RESULTS
  Two hundred and seventy-three patients were admitted to hospital. The age-related population incidences of hospitalization were as follows: 0-14 years: 111/100,000, 15-64 years: 39/100,000, and ≥ 65 years: 17/100,000. During the first wave (July 2009-August 2009), three patients were admitted - none received treatment in intensive care units (ICUs), during the second wave (October 2009-January 2010), 158 patients were admitted - nine received treatment in ICUs, and during the third wave (December 2010-February 2011), 112 patients were admitted - 25 received treatment in ICUs. Fourteen patients (5%) died within 30 days of diagnosis (median 55 years; range 14-76 years) and additional seven patients (2.6%) died within 365 days (median 25 years; range 1-86 years).
  
  
  CONCLUSIONS
  Patients hospitalized with pandemic influenza A (H1N1)pdm09 were predominantly children and younger adults, and only a few patients were >65 years. The third wave was the most severe taking the number and percentage of patients admitted to ICUs and 30-day mortality into consideration. We observed that the incidence of hospitalizations as well as clinical severity among younger adults did not decline from the second to the third wave.
authors: Department of Infectious Diseases, Aalborg Hospital, Aarhus University Hospital, Aalborg, Denmark. iben.oersted@rn.dk
authors: Iben
authors: I
authors: Ørsted
authors: Mette
authors: M
authors: Mølvadgaard
authors: Hans Linde
authors: HL
authors: Nielsen
authors: Henrik
authors: H
authors: Nielsen
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D003718:Denmark / Q000453:epidemiology; D005260:Female; D006760:Hospitalization; D006801:Humans; D015994:Incidence; D007223:Infant; D053118:Influenza A Virus, H1N1 Subtype / Q000235:genetics / Q000302:isolation & purification*; D007251:Influenza, Human / Q000453:epidemiology* / Q000401:mortality / Q000628:therapy / Q000821:virology; D007362:Intensive Care Units; D008297:Male; D008875:Middle Aged; D058873:Pandemics; D055815:Young Adult
keywords: Epidemiology; ICU; hospitalization; influenza; outcome; pandemic
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