The Underrecognized Burden of Influenza in Young Children

BACKGROUND: The disease burden of influenza infection among children is not well established. We conducted a population-based surveillance of medical visits associated with laboratory-confirmed influenza. METHODS: Eligible children were younger than five years of age, resided in three U.S. counties, and had a medical visit for an acute respiratory tract infection or fever. Nasal and throat swabs were tested for the influenza virus by viral culture and polymerase-chain-reaction assay. Epidemiologic data were collected from parental surveys and chart reviews. Children who were hospitalized were enrolled prospectively from 2000 through 2004. Population-based rates of hospitalizations associated with influenza were calculated. Children who were seen in selected pediatric clinics and emergency departments during two influenza seasons (2002-2003 and 2003-2004) were systematically enrolled. The rates of visits to clinics and emergency departments associated with influenza were estimated. RESULTS: The average annual rate of hospitalization associated with influenza was 0.9 per 1000 children. The estimated burden of outpatient visits associated with influenza was 50 clinic visits and 6 emergency department visits per 1000 children during the 2002-2003 season and 95 clinic visits and 27 emergency department visits per 1000 children during the 2003-2004 season. Few children who had laboratory-confirmed influenza were given a diagnosis of influenza by the treating physician in the inpatient (28 percent) or outpatient (17 percent) settings. CONCLUSIONS: Among young children, outpatient visits associated with influenza were 10 to 250 times as common as hospitalizations. Few influenza infections were recognized clinically.