Respiratory Syncytial Virus Burden in Premature Infants: The Role of Season With and Without RSV Immunoprophylaxis in a Multicenter Study

Objective: To compare the Respiratory Syncytial Virus (RSV) hospitalization burden among 29–34 weeks gestational age (wGA) preterm infants between seasons with and without routine palivizumab prophylaxis, by utilizing the 2021 off-season RSV surge. Methods: This multi-center retrospective study was conducted in 11 medical centers across Israel. We included infants > 1 year-old, with wGA data, hospitalized with RSV infection from November 2017-August 2021. National palivizumab compliance data were collected separately. We compared two periods: in-season (November−March) with routine palivizumab prophylaxis as the reference, and off-season (April−October) without prophylaxis as the primary risk factor. The primary outcome was the proportion of RSV hospitalizations in 29–34 wGA infants relative to total RSV admissions, calculated separately for each period. Secondary outcomes included clinical severity parameters. Results: A total of 3296 infants were admitted during the RSV in-season, and 1044 during the off-season. National palivizumab compliance among eligible infants during the in-season study years was 91%−95%. The proportion of 29−34 wGA infants was significantly higher during the off-season compared to the in-season period (7% vs. 2.1%, p < 0.001). In a multivariable logistic regression model, the odds of hospitalization for 29–34 wGA preemies were 2.6 times higher during the off-season compared to the in-season (95% CI: 1.8–3.9, p < 0.001), independent of demographic covariates. Clinical severity was similar between the two periods. Conclusions: Our results revealed a significantly higher proportion of 29−34 wGA infants hospitalized during seasons without palivizumab prophylaxis compared to seasons with palivizumab prophylaxis. These findings highlight the importance of including 29−34 wGA infants into future RSV immunoprophylaxis recommendations.