Respiratory syncytial virus epidemiology and effectiveness of infant nirsevimab: 2024 results from the Australian...

BACKGROUNDRespiratory syncytial virus (RSV) is a leading cause of morbidity and mortality in young children and older adults. A long-acting anti-RSV monoclonal antibody (nirsevimab) and bivalent pre-fusion F-protein pregnancy vaccine became available to prevent RSV in young children in 2024; two RSV vaccines for adults ≥ 60 years were also available.AIMTo report 2024 RSV epidemiology in Australia, identify risk factors for severe outcomes, and use and effectiveness of RSV immunisation products.METHODSNational sentinel hospital-based RSV surveillance was established in 2024, recruiting hospitalised laboratory-confirmed RSV cases and test-negative controls from 22 sites in a national hospital network (FluCAN-PAEDS).RESULTSBetween April and December 2024, 3,998 subjects (3,415 children; 582 adults) were hospitalised with RSV. Most cases were infants < 12 months (n = 1,534; 38.4%); 1,661 (41.5%) had underlying medical conditions. Children < 6 months, First Nations children, those born preterm or with underlying medical conditions (cardiac, neurological, genetic and metabolic disease/disorders, immunosuppression) were at greatest risk of severe outcomes. Severe outcomes were more frequent in adults with malignancy, respiratory or cardiac disease. Nirsevimab effectiveness against hospitalisation in infants < 12 months in the two Australian jurisdictions with population-wide immunisation programmes was 83.1% (95% CI: 67.4-91.3). RSV vaccine use (pregnancy; adults ≥ 60 years) was limited, precluding effectiveness assessments.CONCLUSIONNational surveillance enabled timely 2024 data collection with the capability to evaluate effectiveness of immunisation products preventing RSV. Nirsevimab demonstrated comparable effectiveness to that in the northern hemisphere, informing Australia's 2025 strategy. Evaluation to assess the impact of more widespread uptake of RSV prevention products continues.