title: Virologically confirmed population-based burden of hospitalization caused by influenza A and B among children in Hong Kong.
abstract: BACKGROUND
  We sought to determine the virologically confirmed hospitalization rates associated with influenza virus infection among Hong Kong children.
  
  
  METHODS
  Patients <18 years of age who lived on Hong Kong Island (a separate island within Hong Kong) and were admitted to either of the only 2 public hospitals on the island for a febrile acute respiratory infection on 1 fixed day of the week in each hospital from October 2003 through September 2006 were prospectively recruited. These 2 hospitals together accounted for 72.5% of all general pediatric admissions in Hong Kong Island with a known population denominator. Nasopharyngeal aspirates were obtained from all recruited patients and were tested for influenza A and influenza B viruses by direct antigen detection and culture.
  
  
  RESULTS
  All cases of influenza A during 2003-2004 were caused by H3N2 virus, whereas 85.7% of cases during 2004-2005 were due to H3N2 virus, and 93.5% during 2005-2006 were due to H1N1 virus. During 2004-2005, infants <1 year of age had the highest rate of hospitalization for influenza A (103.8 cases per 10,000 population), whereas children 1 year of age had the highest rate of hospitalization during the other 2 seasons (95.5 and 54.6 cases per 10,000 population during 2003-2004 and 2005-2006, respectively). A protection rate of 25%, presumably attributable to maternal antibodies, was seen in infants <1 year of age who were hospitalized during 2003-2004 with infection due to an H3N2 virus that had been in circulation. The hospitalization rates for influenza B were highest among children 2-4 years of age.
  
  
  CONCLUSIONS
  This population-based study of hospitalizations due to virologically confirmed influenza demonstrated a very high burden of disease among young children in Hong Kong. The morbidity varied with virus type, subtype, and antigenic variants.
authors.affiliation: Department of Paediatrics and Adolescent Medicine, The University of Hong Kong.
authors.forename: Susan S
authors.initials: SS
authors.lastname: Chiu
authors.forename: Kwok-Hung
authors.initials: KH
authors.lastname: Chan
authors.forename: Hong
authors.initials: H
authors.lastname: Chen
authors.forename: Betty W
authors.initials: BW
authors.lastname: Young
authors.forename: Wilina
authors.initials: W
authors.lastname: Lim
authors.forename: Wilfred Hing Sang
authors.initials: WH
authors.lastname: Wong
authors.forename: Yu Lung
authors.initials: YL
authors.lastname: Lau
authors.forename: J S Malik
authors.initials: JS
authors.lastname: Peiris
mesh_terms: D000293:Adolescent; D002648:Child; D002675:Child, Preschool; D005260:Female; D006723:Hong Kong / Q000453:epidemiology; D006760:Hospitalization / Q000706:statistics & numerical data*; D006801:Humans; D015994:Incidence; D007223:Infant; D053118:Influenza A Virus, H1N1 Subtype / Q000302:isolation & purification*; D053122:Influenza A Virus, H3N2 Subtype / Q000302:isolation & purification*; D009981:Influenza B virus / Q000302:isolation & purification*; D007251:Influenza, Human / Q000453:epidemiology* / Q000473:pathology / Q000821:virology*; D008297:Male; D009305:Nasopharynx / Q000821:virology