Effects of Chronologic Age and Young Child Exposure on Respiratory Syncytial Virus Disease among US Preterm Infants Born at 32 to 35 Weeks Gestation.

Objective: To estimate the incidence of respiratory syncytial virus (RSV) disease as a function of chronologic age and exposure to young children in US preterm infants.

Methods: In the RSV Respiratory Events among Preterm Infants Outcomes and Risk Tracking (REPORT) study, preterm infants born at 32-35 weeks gestational age (wGA) were enrolled from 188 US clinics and followed September-May of 2009-2010 or 2010-2011. Infants with medically-attended acute respiratory illness had nasal/pharyngeal swabs collected for viral testing. Results of RSV tests conducted during routine clinical care were also collected. Event rates during November-March were modeled as a function of chronologic age and birth month using Poisson regression and adjusting for other covariates. Rates were calculated overall and for infants with and without exposure to young siblings or daycare attendance. Of 3317 infants screened, 1646 were enrolled as a consecutive sample. Infants with chronic lung disease of prematurity, hemodynamically significant congenital heart disease, life expectancy <6 months, or receiving or being considered for RSV immunoprophylaxis were excluded. 84% of patients completed the study. Demographics of the enrolled cohort were generally similar to those of US infants born at 32-35 wGA; infants 32-34 wGA, Hispanic infants, and infants of less-educated mothers were under-represented.

Results: Among 1642 evaluable infants, outpatient RSV lower respiratory illness incidence was highest at older ages, whereas RSV hospitalization and intensive care unit (ICU) admission were highest at younger ages. In all instances, young child exposure was associated with higher RSV incidence. The highest RSV hospitalization and ICU rates occurred among February-born infants with young child exposure, at 19.0 (95% CI, 13.5-27.0) and 6.5 (95% CI, 5.6-7.6) per 100 infant-seasons, respectively.

Conclusions: Preterm infants have a substantially elevated risk of RSV disease. Young age and exposure to other young children identify those at greatest risk of severe RSV disease.

Trial Registration: Clinicaltrials.gov: NCT00983606.