title: Influenza epidemiology among hospitalized children in Stockholm, Sweden 1998-2014.
abstract: Influenza remains a common reason for the hospitalization of children. There is a need for long term studies that are also population based. We describe the epidemiology of severe influenza in a defined population 1998-2014.
  
  
  
  Retrospective study of annually collected data of virologically confirmed influenza in hospitalized children 0-17 years living in the catchment area (230,000 children). We gathered information about comorbidity and complications from case records, and compared Influenza A, B and A(H1N1)pdm09 with respect to these factors.
  
  
  
  A total of 922 children with influenza were hospitalized. The mean rate remained unchanged at 22.5-24.2 per 100,000 children per year. There were two major outbreaks: influenza A(H3N2) in 2003-2004 and the A(H1N1) pandemic in 2009-2010. The proportion of children with influenza B increased from 8% during the first half of the study period to 28% during the second half. The highest admission rate was found in children <3 months of age, 169 per 100,000. Children with influenza B were older than those with influenza A. Comorbidity was found in 34%, complications in 41%, and 11% needed intensive care management. The mortality rate was 0.17/100,000 children.
  
  
  
  Influenza remains an important reason for the hospitalization of children, especially during the first years of life. The increasing proportion of influenza B may have to be considered when recommending influenza vaccines.
authors.affiliation: Astrid Lindgren Children's Hospital, SE-171 76 Stockholm, Sweden. Electronic address: rutger.bennet@karolinska.se.
authors.forename: Rutger
authors.initials: R
authors.lastname: Bennet
authors.affiliation: Astrid Lindgren Children's Hospital, SE-171 76 Stockholm, Sweden.
authors.forename: Johan
authors.initials: J
authors.lastname: Hamrin
authors.affiliation: Clinical Microbiology, Karolinska University Hospital, SE-171 76 Stockholm, Sweden.
authors.forename: Benita Zweygberg
authors.initials: BZ
authors.lastname: Wirgart
authors.affiliation: Clinical Microbiology, Karolinska University Hospital, SE-171 76 Stockholm, Sweden.
authors.forename: Maria Rotzén
authors.initials: MR
authors.lastname: Östlund
authors.affiliation: Department of Communicable Disease Control and Prevention, Stockholm County Council, SE-118 91 Stockholm, Sweden; Department of Medicine, Unit of Infectious Diseases, Karolinska University Hospital, SE-171 76 Solna, Sweden.
authors.forename: Åke
authors.initials: Å
authors.lastname: Örtqvist
authors.affiliation: Astrid Lindgren Children's Hospital, SE-171 76 Stockholm, Sweden.
authors.forename: Margareta
authors.initials: M
authors.lastname: Eriksson
mesh_terms: D000293:Adolescent; D017677:Age Distribution; D002648:Child; D002673:Child, Hospitalized; D002675:Child, Preschool; D005260:Female; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D053118:Influenza A Virus, H1N1 Subtype; D053122:Influenza A Virus, H3N2 Subtype; D009981:Influenza B virus; D007251:Influenza, Human / Q000453:epidemiology*; D015278:Intensive Care Units, Pediatric; D008297:Male; D012189:Retrospective Studies; D013548:Sweden / Q000453:epidemiology
keywords: Children; Comorbidity; Complications; Epidemiology; Influenza