Effectiveness and impact of nirsevimab in Chile during the first season of a national immunisation strategy against RSV (NIRSE-CL): a retrospective observational study

Nirsevimab for the prevention of respiratory syncytial virus (RSV) was introduced in some countries in the northern hemisphere in 2023. Chile was the first to implement a universal strategy in the southern hemisphere. We aimed to evaluate the effectiveness and impact of nirsevimab during the 2024 RSV season in Chile.

Roll-out of the strategy began on April 1, 2024, and ended on Sept 30, 2024, targeting infants born between April 1, 2024, and Sept 30, 2024 (seasonal newborn cohort), and infants born between Oct 1, 2023, and March 31, 2024 (catch-up cohort). Using historical surveillance and hospital discharge data from ten hospitals that perform universal RSV testing for all patients admitted due to lower respiratory tract infection (LRTI), we identified a set of ICD-10 codes most closely related to RSV admissions during the 2019, 2022, and 2023 RSV seasons. These codes were applied to a national database of three consolidated nationwide government registries to identify RSV-related LRTI hospitalisations (primary endpoint) among infants who received or did not receive nirsevimab. Secondary endpoints were RSV-related intensive care unit (ICU) admission, all-cause LRTI hospitalisation, and all-cause hospitalisation occurring at least 7 days after birth. Nirsevimab effectiveness was estimated using a stratified Cox proportional hazards model, calculated as (1 – hazard ratio) multiplied by 100, with 95% CIs. We also assessed the impact of nirsevimab by estimating, compared with a counterfactual scenario in which nirsevimab was never introduced, the averted number and relative reduction of cases, and the number needed to immunise to avoid one case. This study was registered with ClinicalTrials.gov, NCT06511687 (completed).

Data for 157 709 infants with complete records were extracted from the consolidated database. After excluding 1247 infants with missing or corrupt data and 2289 infants whose immunisation status could not be determined, 154 173 infants were included in the primary analysis. The median age of infants was 6·27 months (IQR 3·20–9·17). 76 045 (49·32%) infants were female and 78 128 (50·68%) were male. 145 087 infants were immunised by the end of the strategy roll-out, with 72 246 (49·79%) in the catch-up cohort and 72 841 (50·21%) in the seasonal cohort. After controlling for age, sex, geographical area, and weeks of gestational age, combined effectiveness of nirsevimab (for catch-up and seasonal cohorts) against RSV-related LRTI hospitalisations was 76·41% (95% CI 72·57–79·72), against RSV-related ICU admissions was 84·94% (79·47–88·95), against all-cause LRTI hospitalisations was 66·50% (61·97–70·50), and against all-cause hospitalisations was 47·90% (44·35–51·21). We estimated a relative reduction of 77·46% in RSV-related LRTI hospitalisations, 30·05 averted cases per 1000 infants, and a number needed to immunise to prevent one RSV-related LRTI hospitalisation of 35.

Chile's nirsevimab immunisation strategy significantly reduced RSV-related LRTI hospitalisations and more severe cases requiring intensive care. Our findings indicate a broader public health impact, with reductions also observed in all-cause LRTI hospitalisations. These results might encourage other countries to advance RSV prevention efforts.