Incidence of Influenza-related Hospitalizations in Different Age Groups of Children in Finland A 16-year Study

Background: Children are frequently hospitalized with influenza-associated illnesses. Few virologically confirmed population-based studies of pediatric hospitalizations performed during several consecutive seasons are available. Methods: This 16-year retrospective study consisted of all children <= 16 years of age who were treated for virologically confirmed influenza at the Department of Pediatrics, Turku University Hospital, Finland, between July 1, 1988 and June 30, 2004. Calculation of the population-based rates of hospitalization in different age cohorts was based on children (n = 69,068) who lived within the 38 municipalities whose acute pediatric care was provided solely by Turku University Hospital. Results: During the study period, 401 children were hospitalized with virologically confirmed influenza. The average annual incidences of influenza-related hospitalizations were highest among children <6 months (276 [95% confidence interval, 220-336] per 100,000) and 6 to 11 months (173 [95% confidence interval, 129-220] per 100,000) of age. For both influenza A and B, the rates of hospitalization were highest among children younger than 1 year of age. Influenza A accounted for 82% and influenza B for 18% of all hospitalizations. A total of 40 (10.0%) children received treatment at the intensive care unit. Of all 401 children with confirmed influenza infection, only 216 (53.9%) had a discharge International Classification of Diseases code related to influenza. Conclusions: The high incidence of influenza-associated hospitalization among infants less than 6 months of age underscores the need to find effective ways to prevent influenza in this age group, in which influenza vaccines are not currently licensed for use.