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Updated: Mar 7, 2026
An Improved and High Throughput Respiratory Syncytial Virus RSV Micro-neutralization Assay
Published on: January 26, 2019
Two-season effectiveness of a single nirsevimab dose against RSV hospitalisation in healthy term-born infants: a
Olivier Núñez1,2, Juan Juaneda3, Montserrat Martinez-Marcos4
1Consortium for Biomedical Research in Epidemiology and Public Health (CIBERESP), Madrid, Spain.
Insights
Nirsevimab significantly reduced respiratory syncytial virus (RSV) hospitalizations in infants over two seasons. This immunization proved effective, decreasing RSV hospitalizations by two-thirds in the first two years of life.
Area of Science:
- Pediatrics
- Immunology
- Public Health
Background:
- Spain recommended nirsevimab for infants born after April 1, 2023, for catch-up or at-birth immunization.
- Respiratory syncytial virus (RSV) poses a significant health risk to infants.
Purpose of the Study:
- To estimate the effectiveness of a single nirsevimab dose against RSV hospitalizations in healthy term-born infants over two seasons.
- To evaluate nirsevimab's efficacy for both catch-up and at-birth immunization strategies.
Main Methods:
- Target trial emulation using case-control data from infants born between April 2023 and March 2024.
- Matched controls were selected based on province and birth date.
- Inverse-probability-weighted conditional logistic regression was used to estimate causal effectiveness.
Main Results:
- Two-season effectiveness for catch-up and at-birth immunization was 64% and 67%, respectively.
- First-season effectiveness was high (78% and 84%), but second-season effectiveness was low or null (-8% and 20%).
- No rebound in RSV hospitalization rates was observed in the second season among immunized children.
Conclusions:
- Nirsevimab is an effective long-term public health intervention for reducing RSV hospitalizations in infants.
- While second-season effectiveness may be underestimated due to survivor bias, the overall impact is substantial.
- The immunization did not lead to a rebound in RSV hospitalizations in the subsequent season.
Abstract:
BACKGROUNDIn autumn 2023, Spain recommended nirsevimab to all infants born after 1 April 2023, as catch-up or at-birth immunisation.AIMWe estimated effectiveness of a single nirsevimab dose against respiratory syncytial virus (RSV) hospitalisations throughout two seasons in healthy term-born infants.METHODSCases were children born 1 April 2023 through 31 March 2024 after 35 gestation weeks without major comorbidities and hospitalised for RSV infection between 2023 immunisation campaign onset and 31 March 2025. We selected four healthy population-density controls per case, matched by province and birth date. Using target trial emulation, causal per-protocol effectiveness was estimated for catch-up (within 30 days of 2023 campaign onset) and at-birth immunisation (within 14 days of life) through cloning, censoring and inverse-probability-weighted conditional logistic regression.RESULTSWe included 235/905 cases/controls for catch-up and 334/1,292 cases/controls for at-birth immunisation (first season), and 188/713 cases/controls for catch-up and 328/1,269 cases/controls for at-birth immunisation (second season). Two-season effectiveness was 64% (95% confidence interval (CI): 52-72) and 67% (95% CI: 59-74) for catch-up and at-birth immunisation, respectively, compared with 78% (95% CI: 70-84) and 84% (95% CI: 79-88) during first season and -8% (95% CI: -88 to 38) and 20% (95% CI: -21 to 46) during second season.CONCLUSIONNirsevimab was an effective long-term population-level intervention, decreasing RSV hospitalisations by two-thirds during the first two seasons of life. Effectiveness during second season was low or null, although it may be underestimated due to unavoidable survivor bias. The RSV hospitalisation rate among immunised children did not rebound in the second season.
