Effectiveness of nirsevimab in preventing RSV-hospitalisation among young children in Western Australia 2024

BACKGROUND: Respiratory Syncytial Virus (RSV) causes a significant burden of illness for children under 2 years of age. Nirsevimab, a long-acting monoclonal antibody, was registered for RSV prevention in Australia in 2023. In April 2024, Western Australia (WA) launched the country's first state-wide nirsevimab program for all infants and high-risk children entering their second RSV season. This study describes the effectiveness of nirsevimab against RSV hospitalisation over a single epidemic season. METHODS: Between April and October 2024, children hospitalised with laboratory-confirmed RSV-associated acute respiratory infection (ARI) and test-negative controls were enrolled from three hospitals in WA. Demographic variables, medical risk factors, symptoms and outcomes were assessed. Nirsevimab effectiveness in preventing RSV-associated hospitalisation was estimated. RESULTS: Over 7 months, 284 children eligible for nirsevimab were enrolled including 184 RSV positive cases and 100 controls. Coverage of nirsevimab in RSV cases was 22.8% and 60.0% in controls. The overall adjusted effectiveness of nirsevimab against RSV-associated ARI hospitalisation was 88.2% (95% CI: 73.5, 94.7). RSV infection occurred in 42 (22.8%) children who had received nirsevimab; there was no significant difference in RSV illness severity among those immunised and unimmunised. CONCLUSION: Nirsevimab was highly effective at preventing RSV-associated ARI hospitalisation in young children in the southern hemisphere. SUMMARY: This study is the first Australian study to provide nirsevimab effectiveness estimate against RSV hospitalisation over a single epidemic season. The adjusted estimate of nirsevimab effectiveness against RSV-associated ARI hospitalisation was 88.2%, similar to those reported from Northern Hemisphere countries.