Respiratory Syncytial Virus Prophylaxis for Premature Infants: Insights from a Multicenter Out-of-Season Study

Objective: To elucidate the impact of lack of Palivizumab administration during off-season RSV periods on the RSV-related hospitalization among 29-34 weeks of gestational age (wGA) preterm infants. We utilized the summer of 2021 off-season RSV surge during the COVID19 pandemic. Methods: : This multi-center retrospective observational study was conducted in 11 medical centers across Israel. We included infants > 1 year-old hospitalized with RSV infection between November 2017-August 2021. Patients were categorized into an in-season admissions group (November-March) and off-season admissions group (April-October). The primary outcome was the proportion of RSV hospitalizations among 29-34wGA infants in relation to total RSV admissions during the same season. Secondary outcomes included clinical severity parameters. Results: A total of 3,296 infants were admitted during the RSV season, and 1,044 during the off-season. The proportion of 29-34wGA preemies was significantly higher in the off-season group compared to the in-season group (7% vs 2.1%, p<0.001). In the off-season group, we observed a significantly higher proportion of Jewish individuals (80.1% vs. 59.2%, p=0.001) and higher socioeconomic status (55.1% vs. 45.1%, p<0.001) compared to the in-season group. In the multivariable logistic-regression model, off-season hospitalization odds for 29-34 wGA preemies were significantly higher (2.6-fold, 95% CI:1.8-3.9, p<0.001) compared to in-season, irrespective of demographic covariates. Similar clinical severity parameters was observed between the groups. Conclusions: Our results revealed a significantly higher proportion of infants born at 29-34 wGA among those hospitalized during off-season periods compared to in-season periods. These findings highlight the importance of including 29-34 wGA infants into future RSV immunoprophylaxis recommendations.