Incidence of Medically Attended Respiratory Syncytial Virus and Influenza Illnesses in Children 6–59 Months Old During Four Seasons

Background. Respiratory syncytial virus (RSV) and influenza are significant causes of seasonal respiratory illness in children. The incidence of influenza and RSV hospitalization is well documented, but the incidence of medically attended, laboratory-confirmed illness has not been assessed in a well defined community cohort. Methods. Children aged 6–59 months with medically attended acute respiratory illness were prospectively enrolled during the 2006–2007 through 2009–2010 influenza seasons in a Wisconsin community cohort. Nasal swabs were tested for RSV and influenza by multiplex reverse-transcription polymerase chain reaction. The population incidence of medically attended RSV and influenza was estimated separately and standardized to weeks 40 through 18 of each season. Results. The cohort included 2800–3073 children each season. There were 2384 children enrolled with acute respiratory illness; 627 (26%) were positive for RSV and 314 (13%) for influenza. The mean age was 28 months (standard deviation [SD] = 15) for RSV-positive and 38 months (SD = 16) for influenza-positive children. Seasonal incidence (cases per 10 000) was 1718 (95% confidence interval [CI], 1602–1843) for RSV and 768 (95% CI, 696–848) for influenza. Respiratory syncytial virus incidence was highest among children 6–11 (2927) and 12–23 months old (2377). Influenza incidence was highest (850) in children 24–59 months old. The incidence of RSV was higher than influenza across all seasons and age groups. Conclusions. The incidence of medically attended RSV was highest in children 6–23 months old, and it was consistently higher than influenza. The burden of RSV remains high throughout the first 2 years of life.