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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.

In this study, we validated the Centers for Disease Control and Prevention's use of a 10% threshold of median proportion of positive laboratory tests (median proportion positive (MPP)) to identify respiratory syncytial virus (RSV) seasons against a standard based on hospitalization claims. Medicaid fee-for-service recipients under 2 years of age from California, Florida, Illinois, and Texas (1999-2004), continuously eligible since birth, were categorized for each week as high-risk or low-risk with regard to RSV-related hospitalization based on medical and pharmacy claims data and birth certificates. Weeks were categorized as on-season if the RSV hospitalization incidence rate in high-risk children exceeded the seasonal peak of the incidence rate in low-risk children. Receiver operating characteristic (ROC) curves were used to measure the ability of MPP to discriminate between on-season and off-season weeks as determined from hospitalization data. Areas under the ROC curve ranged from 0.88 (95% confidence interval: 0.83, 0.92) in Illinois to 0.96 (95% confidence interval: 0.94, 0.98) in California. Requiring at least 5 positive tests in addition to the 10% MPP threshold optimized accuracy, as indicated by minimized root mean square errors. The 10% MPP with the added requirement of at least 5 positive tests is a valid method for identifying clinically significant RSV seasons across geographically diverse states.