Epidemiology of Respiratory Infections in Young Children

BACKGROUND: The New Vaccine Surveillance Network, funded by the Centers for Disease Control and Prevention, was established to provide longitudinal data to help measure the impact of vaccines that decrease the burden of acute respiratory illness in children younger than 5 years of age. Currently 140,000 children younger than 5 years of age, nearly 1% of the U.S. population, are under surveillance in 3 urban counties, which include Nashville, TN, Rochester, NY, and Cincinnati, OH. Prospective, active, population-based surveillance of children hospitalized with respiratory symptoms or fever began in 2000, and outpatient surveillance began in 2002 in selected winter months. RESULTS: During the first year of surveillance, the admission rate for acute respiratory illness/fever in children younger than 5 years of age in the surveillance areas was 180 per 10,000 children. In 61% of these hospitalizations, a respiratory virus was identified. Respiratory syncytial viruses, influenza viruses and parainfluenza viruses were identified in 30%, and other respiratory viruses were identified in 36%. Approximately one-third of children hospitalized had identified high risk conditions, primarily asthma (24%). About one-half of the children hospitalized were younger than 6 months of age. Polymerase chain reaction doubled the diagnostic yield of culture for respiratory syncytial viruses, influenza and parainfluenza virus combined. Preliminary data on influenza admissions during the 4 years indicated considerable variation in admission rates by season and site. Annually the mean admission rates during the study period were 43 per 10,000 children younger than 6 months of age, 9 per 10,000 children 6-23 months of age and 4 per 10,000 children 24-59 months of age. CONCLUSION: These data provide ongoing surveillance of hospitalizations and other medically attended visits for respiratory viral illness in children younger than 5 years of age. Evaluation of influenza vaccine effectiveness, the epidemiology of respiratory viruses in children and the impact of the pediatric influenza vaccination program are in progress.