Respiratory syncytial virus (RSV) is a leading cause of hospitalizations for acute lower respiratory tract infection (LRTI) in infants. In Australia approximately 1.5% of infants are hospitalized due to RSV, 80% of whom are born full-term and are otherwise healthy. Two randomized trials have used a seasonal implementation strategy to show nirsevimab, a long-acting monoclonal antibody, has sustained efficacy against RSV LRTI hospitalizations in the first 150-180 days after administration. Nirsevimab has been approved in many countries for the prevention of RSV-LRTI in neonates and infants. However, the protection offered by nirsevimab beyond 180 days remains unknown. Queensland is a large Australian state spanning the tropical and sub-tropical climate zones, where RSV circulates year-round. The Queensland government publicly funded nirsevimab for all infants at birth from 15 April 2024. The aim of this study is to estimate the duration of effectiveness of nirsevimab against RSV-related hospitalizations.

A case-control study will be conducted using routinely collected linked data. Cases will be children born in Queensland from 15 April 2024 to 14 April 2025 who are hospitalised with an RSV-related condition prior to 14 April 2026. Controls will be drawn from the set of infants who are admitted to the same hospital in a 5:1 ratio, and matched on age and sex using the Queensland Perinatal Data Collection. Nirsevimab receipt will be extracted from hospital's electronic medical records and linked to hospitalization data by the Queensland Health Data Linkage Unit. Duration of protection against RSV-related hospitalisation due to nirsevimab will be assessed using multivariable logistic regression model accounting for matching and adjusting for confounding variables.

This case-control study will determine the level and duration of protection offered by nirsevimab in a region with year-round RSV circulation and inform future prevention strategies. Interim analysis is expected to be available at the time of the conference, allowing for early dissemination of this first evidence about nirsevimab duration of protection beyond 180 days.

Funding: From Sanofi and AstraZeneca through a collaboration grant.

Inclusion Criteria: Any infant born between 15 April 2024 and 30 April 2025 and hospitalized for any reason between 15 April 2024 and 30 April 2026.

Exclusion Criteria: Infants who admitted to hospitals with an elective admission. ((i.e. admissions that are planned ahead of time, such as scheduled surgeries or other pre-arranged medical procedures).

Our study population will consist of all infants born in Queensland between 15 April 2024 and 30 April 2025 and hospitaliszed for any reason between 15 April 2024 and 30 April 2026.

Cases will be All infants born in Queensland between 15 April 2024 and 30 April 2025 who were admitted to any hospital in Queensland (public and private) between 15 April 2024 and 30 April 2026 with a respiratory related principle or other diagnosis (ICD codes J21.0, J20.5, J12.1, B97.4). Controls will be defined as infants admitted to the same hospital that do not have a diagnosis of (ICD codes J21.0, J20.5, J12.1, B97.4) and that the admission is not an elective admission.

For each included case, five controls (where possible) matched on month of birth and sex will be requested to enable appropriate comparison.