Estimating the impact of Western Australia's first respiratory syncytial virus immunisation program for all infants: A mathematical modelling study.

Background: The Australian Therapeutic Goods Administration approved the use of nirsevimab, a long-acting monoclonal antibody for the prevention of Respiratory Syncytial Virus (RSV), in November 2023. Western Australia (WA) implemented a combination of nirsevimab administration strategies designed to protect all infants starting in April 2024, before the epidemic season. We developed a dynamic transmission model to predict the impact of WA's RSV immunisation program on infant hospitalisations.

Methods: Our model is calibrated to monthly-aggregated hospitalisation data of children aged <5 years in temperate WA from 2015 to 2019. The model was extended to capture administration of nirsevimab at varying coverage levels for different age and risk groups, using efficacy trial data to parameterise the simulation of nirsevimab protection at a population-level from the time of administration.

Results: We predict that under the observed coverage levels, approximately one-third of RSV-hospitalisations across temperate WA in under 2-year-olds will be averted by the nirsevimab infant immunisation program. We show that immunisation of newborns between April and September has the largest impact, potentially averting up to 28 % of RSV-hospitalisations in under 2-year-olds. Extending the program to immunise all children in their second year, could avert approximately half of RSV-hospitalisations of young children in temperate WA.

Conclusions: A comprehensive seasonal infant immunisation program using nirsevimab, such as the one implemented in WA, can greatly reduce the RSV-hospitalisation burden in young children, with immunisation at birth prior to and during the yearly epidemic having the most impact on hospitalisations.