Influenza is not a reportable disease, and knowledge of its true prevalence in children is imperfect. To fill in that blank, the Centers for Disease Control and Prevention (CDC) set up the New Vaccine Surveillance Network (NVSN) to prospectively test children younger than 5 years in three counties who had a medical visit for an acute respiratory tract infection or fever between 2000 and 2004. Nasal and throat swabs were tested for flu by viral culture and polymerase chain reaction assay. The study, published recently in the New England Journal of Medicine (2006;355:31) included children who were hospitalized in those years, as well as children seen in selected pediatric clinics and emergency departments during the 2002-2003 and 2003-2004 flu seasons-a period when vaccination was "encouraged" for all children 6 to 23 months but not yet routinely recommended.
Study data show an average annual rate of hospitalization associated with laboratory-confirmed influenza of 0.9/1,000 children. Younger children ran the greatest risk: Nearly half of hospitalized children were younger than 6 months, and 80% were younger than 24 months. The estimated burden of outpatient visits was 50 clinic visits and six ED visits for every 1,000 children during the first season, and 95 clinic visits and 27 ED visits for every 1,000 in the second season. These numbers aren't out of line with earlier reports that were derived from retrospective studies, and they corroborate the existence of a substantial burden of disease among young children.