Appropriateness of age thresholds for respiratory syncytial virus immunoprophylaxis in moderate-preterm infants: a cohort study.

IMPORTANCE Recommendations concerning the appropriate age threshold for respiratory syncytial virus (RSV) prophylaxis in moderate-preterm infants are highly debated. OBJECTIVE To determine the age at which moderate-preterm infants' risk of RSV hospitalization has decreased to the risk observed in low-risk term infants. DESIGN, SETTING, AND PARTICIPANTS Retrospective cohort study of Florida and Texas Medicaid fee-for-service billing records matched to birth certificates from Medicaid beneficiaries aged 0 to 12 months with a sibling younger than 5 years and without other indications for RSV prophylaxis between January 1, 1999, and December 31, 2004. EXPOSURES For each state, we used discrete time survival analysis to develop age trend models for RSV hospitalizations for 2 groups: moderate-preterm infants (32-34 weeks' gestational age) and term infants (37-41 weeks' gestational age). MAIN OUTCOMES AND MEASURES Age at which preterm infants' risk of RSV hospitalization equaled the risk for term infants at age 1 month. RESULTS Our cohort included 247,566 eligible infants with 5322 RSV hospitalizations. Preterm status doubled the risk for RSV hospitalization in both Florida (odds ratio = 2.41; 95% CI, 1.85-3.12) and Texas (odds ratio = 1.94; 95% CI, 1.64-2.30). Preterm infants' risk of RSV hospitalization was similar to that for 1-month-old term infants at ages 4.2 months (95% CI, 2.5-5.7) in Florida and 4.5 months (95% CI, 2.8-6.4) in Texas. CONCLUSIONS AND RELEVANCE The age at which moderate-preterm infants showed RSV hospitalization risk similar to their healthy term counterparts supports the more restrictive age thresholds in RSV immunoprophylaxis recommendations. Further studies are warranted to investigate the age-dependent risk of RSV hospitalization in other RSV risk groups.