The Impact of Limited‐Eligibility Nirsevimab on Paediatric Respiratory Syncytial Virus ( RSV ) Infection and Hospitalisations in the Australian Capital Territory: An Observational Cohort Study

AIM: To assess changes in Respiratory Syncytial Virus (RSV) epidemiology and hospitalisations following implementation of a limited-eligibility nirsevimab program. METHODS: This observational cohort study included all infants aged < 2 years residing in our jurisdiction with laboratory-confirmed RSV and/or receipt of an RSV monoclonal antibody between April 2022 and March 2025. Cases were followed for emergency department (ED) presentations and hospital admissions. Primary outcomes were RSV incidence, ED presentation, and hospitalisation rates. RESULTS: We identified 2355 RSV cases, with 1625 infants presenting to ED and 701 hospitalised. Among 308 infants receiving nirsevimab, the most common indication was prematurity; 2.4% experienced breakthrough RSV infection. RSV incidence increased post-nirsevimab (April 2024-March 2025: 89.06/1000 infant-years) compared with the pre-nirsevimab period (April 2022-March 2024: 67.99/1000 infant-years), consistent with local and national trends. Characteristics of hospitalised infants changed in the post-nirsevimab period: age increased (from 9.4 to 10.7 months, p = 0.019), the proportion aged < 3 months decreased (from 27.6% to 19.7%, p = 0.013) and prematurity prevalence nearly halved (from 13.3% to 7.3%, p = 0.041). Clinical characteristics also changed, with an 8-h reduction in length of stay (p = 0.010), and a one fifth reduction in the need for respiratory support (p < 0.001). No significant differences were observed for Aboriginal and Torres Strait Islander infants. CONCLUSIONS: Hospitalisations among infants at greatest risk for severe RSV disease reduced following introduction of the limited-eligibility nirsevimab program. However, Aboriginal and Torres Strait Islander infants remained over-represented in hospital admissions and under-served by the program in its first year, warranting further investigation.