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.