title: Registry-based surveillance of influenza-associated hospitalisations during the 2009 influenza pandemic in Denmark: the hidden burden on the young.
abstract: BACKGROUND
  To follow the impact of the 2009 influenza pandemic in Denmark, influenza surveillance was extended with a system monitoring potentially influenza-associated hospitalisations.
  
  
  RESULTS
  National administrative data from 2004-2010 from the automatic reporting of all hospital visits and admissions in Denmark (population 5.5 million) were used. In-patient hospitalisations linked to ICD-10 codes for potentially influenza-associated conditions (influenza, viral and bacterial pneumonia, respiratory distress, and febrile convulsion) were aggregated by week and age groups; <5 years, 5-24 years, 25-64 years and ≥65 years. Weekly numbers of influenza-associated hospitalisations were plotted to follow the course of the pandemic. We calculated the total numbers of influenza-associated hospitalisations in each influenza season (week 30 to week 15, the following year). Risk ratios of being admitted with an influenza-associated condition in this season (2009/2010) compared to the previous five seasons (2004/2005-2008/2009) were calculated using binary regression. During the pandemic season, influenza-associated hospitalisations peaked in week 47, 2009. The total number of influenza-associated hospitalisations was 38,273 compared to the median of previous seasons of 35,662 (p = 0.28). The risk ratio of influenza-associated hospitalisations during the pandemic season compared to previous seasons was 1.63 (95%CI 1.49-1.78) for 5-24 year-olds and ranged between 0.98 and 1.08 for the other three age groups.
  
  
  CONCLUSIONS
  The 2009 pandemic influenza did not lead to an overall increase in the number of influenza-associated hospitalisations in Denmark in the 2009/2010 season and could be managed within existing hospital capacity. However, there was a disproportionally large impact on the age group 5-24 years. The influenza-associated hospitalisations during the 2009/2010 pandemic influenza season bore the signature features of historical pandemics: A skewed age-pattern and early out of season transmission.
authors.affiliation: Department of Epidemiology, Statens Serum Institut, Copenhagen, Denmark. widgren.katarina@gmail.com
authors.forename: Katarina
authors.initials: K
authors.lastname: Widgren
authors.forename: Jens
authors.initials: J
authors.lastname: Nielsen
authors.forename: Kåre
authors.initials: K
authors.lastname: Mølbak
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D002648:Child; D002675:Child, Preschool; D003718:Denmark / Q000453:epidemiology; D006760:Hospitalization / Q000706:statistics & numerical data*; D006801:Humans; D015994:Incidence; D007251:Influenza, Human / Q000453:epidemiology / Q000628:therapy*; D038801:International Classification of Diseases / Q000706:statistics & numerical data; D008875:Middle Aged; D058873:Pandemics; D011159:Population Surveillance / Q000379:methods*; D012042:Registries / Q000706:statistics & numerical data*; D012307:Risk Factors; D012621:Seasons; D055815:Young Adult