title: Age distribution of cases of 2009 (H1N1) pandemic influenza in comparison with seasonal influenza.
abstract: BACKGROUND
  Several aspects of the epidemiology of 2009 (H1N1) pandemic influenza have not been accurately determined. We sought to study whether the age distribution of cases differs in comparison with seasonal influenza.
  
  
  METHODS
  We searched for official, publicly available data through the internet from different countries worldwide on the age distribution of cases of influenza during the 2009 (H1N1) pandemic influenza period and most recent seasonal influenza periods. Data had to be recorded through the same surveillance system for both compared periods.
  
  
  RESULTS
  For 2009 pandemic influenza versus recent influenza seasons, in USA, visits for influenza-like illness to sentinel providers were more likely to involve the age groups of 5-24, 25-64 and 0-4 years compared with the reference group of >64 years [odds ratio (OR) (95% confidence interval (CI)): 2.43 (2.39-2.47), 1.66 (1.64-1.69), and 1.51 (1.48-1.54), respectively]. Pediatric deaths were less likely in the age groups of 2-4 and <2 years than the reference group of 5-17 years [OR (95% CI): 0.46 (0.25-0.85) and 0.49 (0.30-0.81), respectively]. In Australia, notifications for laboratory-confirmed influenza were more likely in the age groups of 10-19, 5-9, 20-44, 45-64 and 0-4 years than the reference group of >65 years [OR (95% CI): 7.19 (6.67-7.75), 5.33 (4.90-5.79), 5.04 (4.70-5.41), 3.12 (2.89-3.36) and 1.89 (1.75-2.05), respectively]. In New Zealand, consultations for influenza-like illness by sentinel providers were more likely in the age groups of <1, 1-4, 35-49, 5-19, 20-34 and 50-64 years than the reference group of >65 years [OR (95% CI): 2.38 (1.74-3.26), 1.99 (1.62-2.45), 1.57 (1.30-1.89), 1.57 (1.30-1.88), 1.40 (1.17-1.69) and 1.39 (1.14-1.70), respectively].
  
  
  CONCLUSIONS
  The greatest increase in influenza cases during 2009 (H1N1) pandemic influenza period, in comparison with most recent seasonal influenza periods, was seen for school-aged children, adolescents, and younger adults.
authors: Alfa Institute of Biomedical Sciences, Athens, Greece.
authors: Drosos E
authors: DE
authors: Karageorgopoulos
authors: Evridiki K
authors: EK
authors: Vouloumanou
authors: Ioanna P
authors: IP
authors: Korbila
authors: Anastasios
authors: A
authors: Kapaskelis
authors: Matthew E
authors: ME
authors: Falagas
mesh_terms: D000293:Adolescent; D000328:Adult; D017677:Age Distribution; D000368:Aged; D002648:Child; D002675:Child, Preschool; D006760:Hospitalization / Q000706:statistics & numerical data; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D053118:Influenza A Virus, H1N1 Subtype / Q000472:pathogenicity*; D007251:Influenza, Human / Q000175:diagnosis / Q000453:epidemiology* / Q000401:mortality; D007753:Laboratories; D008875:Middle Aged; D058873:Pandemics / Q000706:statistics & numerical data*; D012621:Seasons; D055815:Young Adult