Incidence of respiratory syncytial virus bronchiolitis in hospitalized infants born at 33–36 weeks of gestational age compared with those born at term: a retrospective cohort study

Objective: The aim was to compare incidences of respiratory syncytial virus (RSV) bronchiolitis in late preterm vs. term infants (33–36 vs. >36 weeks of gestational age (WGA)). Methods: This was a population-based retrospective study including all infants <12 months hospitalized at Soroka medical centre with bronchiolitis between 2004 and 2012. Infants with comorbidities were excluded. RSV bronchiolitis rates were calculated by extrapolating the proportion of positive tests among tested infants. Population denominator for incidence rates was calculated from hospital records. Results: During the study, 374 late preterm and 2948 term infants were hospitalized with bronchiolitis. Out of 229 (61.2%) late preterm infants and 1738 (59%) term infants tested for RSV, 164 (71.6%) and 1266 (72.8%) were positive for RSV respectively. The mean yearly incidences per 1000 children of RSV bronchiolitis hospitalizations of late preterm and term infants were 35.8 ± 13.0 and 19.6 ± 4.1 respectively (p 0.009). During RSV seasons the mean incidence rate ratio between groups was 1.82 (95% CI 1.60–2.08). Duration of hospitalization was 4.8 ± 7.0 and 3.9 ± 4.9 in late preterm and term infants, respectively (p 0.003). Conclusions: Late preterm-born infants (33–36 WGA) had a higher rate of hospitalization for overall and RSV bronchiolitis during the first year of life compared to those born at term.