Incidence of influenza in Finnish children.
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.
Heikkinen
Terho
T
Departments of Pediatrics, Turku University Hospital, Turku, Finland.
Ziegler
Thedi
T
Peltola
Ville
V
Lehtinen
Pasi
P
Toikka
Pia
P
Lintu
Mikko
M
Jartti
Tuomas
T
Juvén
Taina
T
Kataja
Janne
J
Pulkkinen
Jaakko
J
Kainulainen
Leena
L
Puhakka
Tuomo
T
Routi
Taina
T
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