TOPLINE:

A pooled analysis of children from three European countries found that immunisation with nirsevimab during the 2024-2025 season effectively prevented respiratory syncytial virus (RSV)-related hospitalisations in those younger than 24 months; however, the protective effect declined gradually over time since immunisation.

METHODOLOGY:

• Researchers conducted a test-negative case-control pilot study across Belgium, Portugal, and Spain to evaluate the effectiveness of nirsevimab against laboratory-confirmed RSV infection among children hospitalised for severe acute respiratory infection during the 2024-2025 season.
• A total of 2201 children younger than 24 months were included, of whom 791 (42.6% girls) who tested positive for RSV were compared with 1410 (42.1% girls) who tested negative for RSV.
• Children were classified as immunised if they received nirsevimab between September 2024 and May 2025 before testing, regardless of the dose or their age or weight at immunisation.

TAKEAWAY:

• The pooled overall immunisation effectiveness was 79% (95% CI, 58%-89%), with an effectiveness of 80% seen in infants aged 0-6 months and 74% seen in children aged 7-23 months.
• The protective effect of nirsevimab declined over time: the effectiveness was 85% within 30 days post-immunisation, which decreased to 78% at 30-89 days and further reduced to 69% at 90-215 days from immunisation.
• Among infants aged 0-6 months, the immunisation effectiveness was 85% within 30 days of immunisation, 78% at 30-89 days, and 53% at 90-215 days from immunisation.

IN PRACTICE:

"If validated in future seasons, our results by time since immunisation underscore the critical role of RSV surveillance for optimising the start of immunisation programme with long-acting monoclonal antibodies, rather than relying on programmatic schedules," the authors wrote.

SOURCE:

This study was led by Camelia Savulescu and Iris Ganser, Epiconcept, Paris, France. It was published online on November 13, 2025, in Eurosurveillance.

LIMITATIONS:

Information on immunisation status was missing for 15% of children, and heterogeneity existed between sites in terms of immunisation recommendations and case definitions. The reduced specificity of the severe acute respiratory infection case definition in young children may have led to an underestimation of immunisation effectiveness.

DISCLOSURES:

This study was funded by the European Centre for Disease Prevention and Control through a Vaccine Effectiveness, Burden and Impact Studies Lot 1 framework contract. One author reported receiving honoraria from Sanofi for a lecture on RSV, and another reported receiving national funding from Belgian Public Services (FPS Santé Publique) for severe acute respiratory infection surveillance.

This article was created using several editorial tools, including AI, as part of the process. Human editors reviewed this content before publication.