Although many infants with respiratory syncytial virus (RSV) infection are hospitalized, most infants are treated as outpatients. Limited data are available on the burden of RSV in outpatient infants.

In a prospective study, we enrolled 431 newborn infants and followed them for a 10-month period (September-June). During each respiratory illness, we examined the infants and obtained nasopharyngeal specimens for the detection of RSV. The parents completed daily symptom diaries throughout the study.

Among 408 active participants, the seasonal incidence rate of RSV illness was 328.4 per 1000 (95% confidence interval [CI] 275.2-389.0). Infants with ≥1 sibling had a 1.9-fold higher incidence of RSV illness than those without (95% CI 1.3-2.8; P = .0007). Acute otitis media developed in 103 (76.9%) of 134 infants with RSV infection, and 95 (70.9%) were treated with antibiotics. Nine (6.7%) infants with RSV were hospitalized, for a seasonal incidence rate of RSV hospitalization of 22.1 per 1000 (95% CI 10.1-41.9).

The outpatient burden of RSV is heavy on infants during the first year of life. Acute otitis media is a frequent complication of RSV, and it should be included in cost-effectiveness analyses of prevention or treatment of RSV infections in infants.