Impact of Nirsevimab on Respiratory Syncytial Virus PCR Test Positivity in Young Infants: A Community‐Level Observational Study in Queensland, Australia

AIM: In April 2024 nirsevimab, a long-acting respiratory syncytial virus (RSV)-specific monoclonal antibody, was made available free of charge to all newborn infants in Queensland. We aimed to explore the community-level impact of a nirsevimab-based prevention program on RSV detection percentages among infants aged ≤ 3 months in Queensland, Australia. METHODS: A retrospective analysis of January 2022-December 2024 laboratory data from a Queensland network. Weekly percentages of RSV-positive tests among infants aged ≤ 3 months (eligible group for nirsevimab) were compared with children aged 24-35 months (ineligible group) after nirsevimab introduction in mid-April 2024, and with comparable age data from 2022 to 2023. RESULTS: Overall, 20 956 RSV polymerase chain reaction tests for both age groups were performed during the study period (2022: 5593; 2023: 4910; 2024: 10 453). Following nirsevimab introduction, the percentage of RSV positive tests declined from 16.0% (370/2316) to 5.8% (51/876) among eligible infants (aged ≤ 3 months), compared to 18.6% (1879/10 098) to 14.6% (1117/7666) among ineligible children (aged 24-35 months). Difference-in-differences analysis showed an absolute risk difference of -6.9 percentage points (95% confidence interval: -13.2 to -4.1, p = 0.03). CONCLUSIONS: We present the first real-world community-based evidence on the impact of nirsevimab upon RSV detection percentages in infants aged ≤ 3 months in Queensland, Australia. The nirsevimab-based prevention program may be contributing to a substantial decline in RSV detection percentages among young infants in the community.