Estimation of Public Health Impact with Use of Nirsevimab During the 2024-2025 RSV Season in the USA: a Modeling...

Respiratory syncytial virus (RSV) is a leading cause of medically attended lower respiratory tract disease (MA-LRTD) and hospitalization among infants in the USA. With the introduction in 2023 of nirsevimab, a long-acting monoclonal antibody for infant RSV prevention, early data on observed uptake provides an opportunity to quantify its public health impact. We hypothesized that the observed nirsevimab immunization coverage rate during the 2024-2025 RSV season substantially reduced the clinical burden of RSV relative to the pre-2023 standard of practice (SoP) and that increasing coverage levels comparable with other routine pediatric immunizations would yield substantial reductions in RSV-associated outcomes.

We employed a previously published and validated static decision model to estimate RSV MA-LRTDs under multiple nirsevimab coverage scenarios. The analysis compared projected RSV burden under the pre-2023 SoP where only high-risk infants receive palivizumab with scenarios reflecting the observed national nirsevimab coverage during the 2024-2025 RSV season (44.7%) and higher hypothetical coverage levels up to universal, 100% coverage among infants not eligible for palivizumab. Outcomes included RSV-related hospitalizations, emergency room (ER) visits, primary care (PC) visits, and RSV-associated deaths. Number needed to immunize (NNI) values were also calculated relative to the pre-2023 SoP.

Under the pre-2023 SoP, the model estimated 523,594 RSV MA-LRTDs among infants during the 2024-2025 RSV season. At the observed nirsevimab coverage of 44.7%, 204,306 RSV MA-LRTDs were averted, including 18,765 hospitalizations (42% reduction), 49,570 ER visits (39%), and 135,942 PC visits (39%). Increasing coverage to 80.4%-aligned with uptake for other routine infant vaccines-was projected to avert 364,204 RSV MA-LRTDs (159,898 additional cases prevented beyond the 44.7% scenario), including 32,404 hospitalizations (73% reduction). Universal coverage (100%) yielded the greatest benefit, averting 451,991 cases (86% reduction) and 39,891 hospitalizations. The number needed to immunize (NNI) was 9 for one MA-LRTD averted, 91 for one hospitalization, and 34 for one ER visit.

Nirsevimab has already achieved substantial reductions in RSV-associated clinical burden at current national coverage levels. Modeled increases in coverage demonstrate large, incremental public health gains with complete universal immunization yielding up to an 86% reduction in RSV MA-LRTDs. These findings highlight the importance of implementing strategies that enhance access, reduce inequities, and support higher, sustained coverage to maximize population-level benefits of RSV prophylaxis.