# P-708. Clinical characteristics and outcomes among critically ill children aged 8 to 24 months with RSV lower respiratory tract infection by nirsevimab eligibility status – October 2023 – April 2025

## TL;DR

The study examines the clinical features and outcomes of critically ill children aged 8 to 24 months with RSV infection, focusing on those eligible for nirsevimab treatment.

## Contribution

The study provides insights into the proportion of nirsevimab-eligible children and the characteristics of those who are not eligible.

## Key findings

- Only 6% of 482 children met nirsevimab eligibility criteria, primarily due to bronchopulmonary dysplasia.
- 34% of ineligible children had underlying conditions like cardiac or neurologic disorders.
- Most children with severe RSV-LRTI were ineligible for nirsevimab despite requiring ICU care.

## Abstract

RSV is the leading cause of infant hospitalization, and some children with underlying conditions are at increased risk of severe RSV during their 2nd RSV season. The Advisory Committee on Immunization Practices recommends nirsevimab for U.S. children in their 2nd season with specified risk factors (Table 1). Criteria for 2nd season eligibility vary by country. Our objectives were 1) to understand the proportion of nirsevimab-eligible children and 2) to describe underlying conditions and outcomes among those not eligible in a cohort of U.S. children with severe RSV lower respiratory tract infection (LRTI).

We enrolled children aged 8−24 months hospitalized across 28 hospitals in 24 states with severe RSV LRTI during their 2nd RSV season during October 30, 2023–April 12, 2024, and October 1, 2024–March 31, 2025. Analytic inclusion required 1) admission to the ICU for ≥24 hours and 2) high flow nasal cannula or noninvasive (continuous positive airway pressure [CPAP] or bilevel positive airway pressure [BiPAP]) or invasive ventilation. Patient characteristics and outcomes were compared by nirsevimab eligibility status using chi-square or exact tests.

Among 482 children with severe RSV, 33 (6%) met 2nd season nirsevimab eligibility: 5 (15%) were American Indian/Alaska Native, 24 (73%) had bronchopulmonary dysplasia (BPD) requiring ongoing medical support, one (3%) had cystic fibrosis with severe lung disease, and 3 (9%) had severe immunocompromise (Table 2). Many had other comorbidities, including 39% and 24% with cardiac and neurologic/neuromuscular disorders, respectively. Among 449 ineligible children, 151 (34%) had at least one underlying condition, and receipt of noninvasive or invasive ventilation was associated with neurologic or neuromuscular disease (p=0.048) (Table 3). Three children received nirsevimab in their 2nd RSV season, including 2 children with BPD and one with chronic lung disease and congenital heart disease.

Most children with ICU admission for severe RSV-LRTI during their 2nd RSV season were ineligible for nirsevimab. Among those who were eligible, most qualified through having BPD. One-third of ineligible children with life-threatening illness had one or more respiratory, cardiac, and neuromuscular comorbidities.

Natasha B. Halasa, MD, CSL-Seqirus: Advisor/Consultant|Merck: Grant/Research Support Adrienne G. Randolph, MD, MSc, Thermo Fisher, Inc.: Advisor/Consultant|UpToDate Inc.: Section editor, Pediatric Critical Care