1943. RSV Prophylaxis for 29-34 Gestational Preemies. Are Current Guidelines Justified? Lessons from an Israeli Multicenter Off-seasonal Study

BACKGROUND: Respiratory syncytial virus (RSV) prophylaxis provided during the winter to infants born at 29–34-week gestational age (wGA) is controversial. We utilized the unprecedented opportunity of high load off-season RSV admissions with no prophylaxis given during the summer of 2021, to assess the impact of withholding palivizumab on the relative burden of hospitalized 29-34 wGA infants. METHODS: This was a national multi-center observational retrospective study in 11 medical centers in Israel. We included infants under 1 year-old, who were hospitalized with RSV between November 2017 and August 2021. Demographics and clinical data were extracted from the medical records. We categorized our cohort into a seasonal admissions group (November- March) when palivizumab was indicated, and an off-season admissions group (April-October) when palivizumab was not indicated. The primary outcome was the proportion of 29-34 wGA hospitalizations in each group. Secondary outcomes included clinical severity parameters. RESULTS: We included 4,340 children in our study; 57% were males, 61% were Jewish, and the median age at admission was 2.9 months (IQR 1.4-6.4). A total of 3,296 infants were admitted during the RSV season, and 1,044 were admitted off-season. The proportion of 29-34 wGA preemies was significantly higher in the off-season admissions group compared to the in season patients (7% vs 2.1%, p< 0.001). We found a significantly higher prevalence of Jewish (77% vs 57%, p=0.001) and higher socioeconomic status (55% vs 45%, p< 0.001) children in the group admitted off-season compared to those in season. Logistic regression analysis revealed that the relative-odds of 29-34 wGA preemies to be hospitalized off-season was 2.67 (95% CI: 1.8-3.9, p< 0.001) fold higher than in season, independent of ethnicity, socioeconomic-status, gender, age, or institution. Clinical severity parameters did not differ significantly between the groups. [Figure: see text] Flow chart of study participants [Figure: see text] The proportion of 29-34 wGA preterm infants out of total admissions categorized into seasonal and out of season RSV infections [Figure: see text] Clinical severity in the subgroup of 29-34 wGA preterm infants CONCLUSION: Our results reveal a significant higher proportion of 29-34 wGA preemies among the children hospitalized during off-season periods, when RSV immunoprophylaxis was not indicated. These findings may support the current broad Israeli policy of administration. DISCLOSURES: All Authors: No reported disclosures