Nirsevimab provides age-specific protection against RSV hospitalisation in the first year of life: a population-level study in Queensland, Australia.

Nirsevimab, a long-acting monoclonal antibody with activity against respiratory syncytial virus (RSV), was introduced into the Queensland paediatric RSV prevention program in 2024. Using Queensland Notifiable Conditions Register (NoCS) data, we found that the infant cohort eligible for the nirsevimab program had lower RSV hospitalisation rates than comparable pre-program cohorts. The population-level impact of nirsevimab was greatest during the first month of life (incidence rate ratio [IRR]: 0.22; 95% confidence interval [95% CI]: 0.16-0.31; representing a 78% reduction in hospitalisation rate), with its magnitude gradually declining thereafter until nine months of age (IRR: 0.77; 95% CI: 0.62-0.96; a 23% reduction), following which the impact of nirsevimab on hospitalisation rate, with increasing age of the infant or child, was no longer statistically significant. An interrupted time-series analysis demonstrated an immediate 63% reduction in RSV hospitalisations following nirsevimab's introduction (IRR: 0.37; 95% CI: 0.23-0.60), showing a substantial population-level impact of the program in Queensland, Australia.