506 - Effectiveness of Nirsevimab and RSVpreF on Respiratory Syncytial Virus Infections Among Infants in Hawaiʻi, 2023-2025: A Prospective Cohort Study.

Publication Number: 3487.506

Kayla G. Lum, Hawaii Pacific Health, Honolulu, HI, United States; Chieko Kimata, Hawaii Pacific Health, Honolulu, HI, United States; Kara Wong Ramsey, University of Hawaii, John A. Burns School of Medicine, honolulu, HI, United States; Rodolfo Bégué, University of Hawaii, John A. Burns School of Medicine, Honolulu, HI, United States

Background: Respiratory syncytial virus (RSV) prophylaxis in infants includes maternal-administration of RSVpreF or infant-administration of Nirsevimab. Studies evaluating Nirsevimab or RSVpreF's effectiveness have been conducted in areas where RSV circulates seasonally, and participants were immunized on a seasonal schedule. In contrast, the state of Hawaii has adopted a unique year-round immunization strategy based on local historical RSV trends.

Objective: This study aimed to estimate the vaccine effectiveness (VE) of RSVpreF or Nirsevimab to prevent RSV-related medical encounters among infants in Hawaii.

Design/Methods: Medicaid-covered infants born within the Hawaii Pacific Health network between December 2023 to March 2025 were identified, as only Medicaid-covered infants were offered Nirsevimab during the newborn hospitalization per hospital policy. Infants who received Palivizumab were excluded. Medically attended RSV events were detected through medical chart review. Comparisons between immunized and non-immunized groups used risk ratios (RR). VE was estimated as (1-RR)*100 where RR=(risk of RSV-positivity among immunized infants)/(risk of RSV-positivity among non-immunized infants).

Results: 2478 infants were identified, of which 1609 (65%) received some form of RSV prophylaxis: 854 (34%) Nirsevimab only, 688 (28%) maternal RSVpreF only, and 67 (3%) both. In the period 0-6 months after birth or after receiving Nirsevimab, there were significant differences in medically-attended RSV events between infants who received only Nirsevimab and non-immunized infants (2.1% vs 5.6%, p< 0.001, RR=0.37). There were no significant differences in medically-attended RSV events between infants who received only RSVpreF and non-immunized infants (3.8% vs. 5.6%, p=0.09, RR=0.67) or between infants who received both forms of RSV prophylaxis and non-immunized infants (3.8% vs. 5.6%, p=0.40, RR=0.50). VE was 63% (confidence interval (CI): 36%-78%) for Nirsevimab in the first 6 months. A seasonality trend was observed with most (74%) medically-attended RSV events occurring October 2024-February 2025.

Conclusion(s): The year-round strategy of Nirsevimab administration in Hawaii showed a decrease in medically attended RSV events, but RSVpreF did not show a decrease in the preliminary results. Analysis is ongoing to include additional regression analysis on risk factors, such as prematurity and NICU stay. Future study is needed to explore the best strategy of effective use of RSV prophylaxis in Hawaii amidst potentially changing trends in seasonality.

Table 1. Demographic Table


Table 2. Risk Ratios and Vaccine Efficacy Across 0-6 Months and 6-12 Months
Only infants who were alive for at least 365 days after birth or receiving Nirsevimab were included in the 6-12 month analysis. Nirsevimab shows a greater VE of 63% in an infantʻs first 180 days but has no effectiveness after 180 days. RSVpreF did not show any effectiveness.

Monthly Distribution of Medically-Attended RSV Events
A total of 207 infants experienced a medically-attended RSV event within this study, of which 90 (43%) were non-immunized, 65 (31%) received Nirsevimab, 46 (22%) received RSVpreF, and 6 (3%) received both immunizations. Most medically-attended RSV events (73%) occurred from October 2024 to February 2025, showing similar seasonal patterns as the continental U.S.