First Year Impact of Nirsevimab on Paediatric Respiratory Syncytial Virus Infection and Hospitalisations in the Australian Capital Territory: An Observational Study

RSV infection is the leading cause for hospitalisation in young children. Treatment of RSV is primarily supportive, and includes supplemental oxygen therapy, ventilatory support, and fluid management. Prevention of paediatric RSV has been limited to monoclonal antibody (MAB) use for selected high-risk infants: palivizumab, a neutralising antibody against RSV, reduces RSV-related hospitalisation and illness severity in high-risk infants when administered monthly throughout the RSV season. Palivizumab was approved in Australia in 1999, however cost effectiveness has curtailed broad access. A novel long-acting monoclonal antibody, nirsevimab, was approved in Australia in November 2023. A single dose provides protection against RSV for the duration of an RSV season, and it is now recommended in preference to palivizumab. Nirsevimab is funded by individual jurisdictions. The Australian Capital Territory (ACT) commenced a nirsevimab program in April 2024, offered to infants born less than 37 weeks' gestation after 31st October 2023, all Aboriginal and Torres Strait Islander babies born after 31st October 2023, and infants with specific medical conditions.

This study aims to describe and compare the incidence of RSV infection, RSV-related ED presentations and hospital admissions, and severity of illness in children <2 years in the ACT, pre- and post- nirsevimab introduction. he pre-nirsevimab period will be defined as 1/4/2022 to 31/3/2024 and the post-nirsevimab period 1/4/2024 to 31/3/2025.