510 - Nirsevimab vs Palivizumab: 3‑Season Analysis of Hospital Admission Rates and LOS for High‑Risk Infants < 20 months in a Federally Qualified Health Center NICU Follow‑Up Clinic

Publication Number: 3491.510

Anne F. Kay, Denver Health Hospital Authority, Denver, CO, United States; Philip Miller, University of Colorado School of Medicine, Denver, CO, United States; Cassana Littler, Denver Health & Hospital Authority; University of Colorado School of Medicine, Morrison, CO, United States

Background: RSV antibody prophylaxis is recommended for children at increased risk for severe RSV disease and helps prevent prolonged hospitalization.

Objective: To compare hospital admission rates and LOS among high‑risk infants in the DH NICU‑Follow‑Up clinic over three respiratory seasons, evaluating the shift from a multi‑dose Palivizumab regimen (5 doses/season) to a single‑dose Nirsevimab strategy.

Design/Methods: RSV‑antibody administration rates were determined from our EHR of high‑risk patients in the DH NICU‑Follow‑Up clinic (Sept 7, 2022-Mar 31, 2025) (Fig. 1). Hospitalization rates and LOS in the DH Pediatric Unit and PICU for primary RSV diagnosis were compared each season per state public‑health definition (Fig. 2).

Results: In the 2022‑23 season, 65 high‑risk NICU follow‑up patients were eligible for Palivizumab. 40% received all doses, 40% received partial doses, and the remaining declined or were lost to follow up (Fig. 1). Among those who received Palivizumab, 5 (19.2%) experienced breakthrough RSV (Fig. 3); 3 required hospitalization. Five families (7.7%) declined prophylaxis, and 2 of these infants were hospitalized. Mean LOS was shorter for those who received Palivizumab (1.60 days ± 1.59, 95% CI) vs those without (8.50 days ± 8.82, 95% CI) (Fig. 1). In the 2023‑24 season, 50 infants were eligible for Palivizumab and Nirsevimab, with 88% completing prophylaxis (Figure 3). Four (10.8%) had breakthrough RSV after receiving 1-4 doses of Palivizumab, but none of the infants receiving Nirsevimab had breakthrough infections (Figure 3). Of the four breakthrough cases, two required hospitalization, with a mean LOS of 0.75 days ± 0.94 (95 % CI) (Fig. 1). In the 2024‑25 season, 68 infants were eligible for Nirsevimab, with 91% completing prophylaxis (Fig. 3). Six (9.7%) had breakthrough RSV, five required hospitalization (Fig. 3), with a mean LOS of 3.67 days ± 2.80 (95% CI) (Fig. 1). Three families (4.4%) declined Nirsevimab, and 2 of them were hospitalized with a mean LOS of 1.00 day ± 1.96 (95 % CI ). Four infants were ineligible for Nirsevimab because their birthing parent had received Abrysvo before delivery; one of those infants was hospitalized for 10 days (Fig. 1).

Conclusion(s): Over three seasons, the overall hospital admission rate for RSV decreased, and switching from multi-dose Palivizumab to single-dose Nirsevimab led to higher completion rates of RSV prophylaxis, increasing from 40.0% to 94.0%. Breakthrough infection rates also dropped from 19.2% to 9.7%. While hospital LOS did not differ significantly, the data support the effectiveness of RSV prophylaxis in reducing breakthrough infections.

Figure 1: Hospital LOS Related to RSV Infection Rates 2022-2025
Figure 1: Hospital LOS Related to RSV Infection Rates 2022-2025

Figure 2: RSV Positive Tests-Inpatient Service 2022-2025
Figure 2: RSV Positive Tests-Inpatient Service 2022-2025

Figure 3: RSV Prophylaxis Completion and Breakthrough Rates 2022-2025
Figure 3: RSV Prophylaxis Completion and Breakthrough Rates 2022-2025