title: Incidence of influenza in Finnish children.
abstract: BACKGROUND
  Influenza is an important cause of respiratory illness in children, but data on virologically confirmed influenza infections in children treated as outpatients are limited.
  
  
  METHODS
  We carried out a prospective cohort study of normal children younger than 13 years (n = 1338) in the winter of 2000 to 2001. During the study period of 32 weeks, the children were examined at the study clinic whenever they had fever or signs of respiratory infection. Nasal swabs were obtained during each episode of infection for determination of the viral etiology of the illness.
  
  
  RESULTS
  The overall attack rate of influenza in the cohort was 18.8%. Influenza viruses were isolated from the children from the beginning of November 2000 through May 2001. Virtually in each week between mid-November and the end of April (a period of 24 weeks), influenza viruses accounted for at least 5% of all respiratory infections in the children. During the peak of the epidemic, the percentage of influenza-positive children exceeded 20%.
  
  
  CONCLUSIONS
  This study confirms the important role of influenza as a cause of acute respiratory infections in children, even in winters of mild or moderate influenza activity. The study also shows that influenza viruses may circulate in the community at substantial levels much longer than previously thought.
authors.affiliation: Departments of Pediatrics, Turku University Hospital, Turku, Finland.
authors.forename: Terho
authors.initials: T
authors.lastname: Heikkinen
authors.forename: Thedi
authors.initials: T
authors.lastname: Ziegler
authors.forename: Ville
authors.initials: V
authors.lastname: Peltola
authors.forename: Pasi
authors.initials: P
authors.lastname: Lehtinen
authors.forename: Pia
authors.initials: P
authors.lastname: Toikka
authors.forename: Mikko
authors.initials: M
authors.lastname: Lintu
authors.forename: Tuomas
authors.initials: T
authors.lastname: Jartti
authors.forename: Taina
authors.initials: T
authors.lastname: Juvén
authors.forename: Janne
authors.initials: J
authors.lastname: Kataja
authors.forename: Jaakko
authors.initials: J
authors.lastname: Pulkkinen
authors.forename: Leena
authors.initials: L
authors.lastname: Kainulainen
authors.forename: Tuomo
authors.initials: T
authors.lastname: Puhakka
authors.forename: Taina
authors.initials: T
authors.lastname: Routi
mesh_terms: D000208:Acute Disease; D000293:Adolescent; D017677:Age Distribution; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D004196:Disease Outbreaks; D005260:Female; D005387:Finland / Q000453:epidemiology; D006801:Humans; D015994:Incidence; D009980:Influenza A virus / Q000302:isolation & purification*; D009981:Influenza B virus / Q000302:isolation & purification*; D007251:Influenza, Human / Q000175:diagnosis / Q000453:epidemiology*; D008297:Male; D011446:Prospective Studies; D012141:Respiratory Tract Infections / Q000453:epidemiology / Q000821:virology; D012307:Risk Factors; D017678:Sex Distribution