TOPLINE:

Nirsevimab immunoprophylaxis conferred sustained protection against severe respiratory syncytial virus (RSV)-related outcomes — including hospital and paediatric ICU admissions — across two consecutive epidemic seasons, with a substantial reduction seen during the first season without increasing the risk for severe disease during the second season.

METHODOLOGY:

• Researchers in Spain conducted a retrospective cohort study using linked clinical data to evaluate the effect of nirsevimab immunoprophylaxis on RSV-associated outcomes in 51,154 infants born in Catalonia between April 2023 and March 2024.
• Immunised infants — those who received nirsevimab during the first immunisation campaign (n = 45,534; median age, 366 days; 51.5% boys) were compared with those who did not receive nirsevimab (n = 5183; median age, 378 days; 51.6% boys).
• Participants were followed up from October 2023 to February 2025 and assessed from the end of the first RSV season through the end of the second RSV season.
• Outcomes were RSV-associated hospital admissions, paediatric ICU admissions, emergency department visits for bronchiolitis, and both RSV-related bronchiolitis and RSV infections recorded in primary care.

TAKEAWAY:

• By the end of the first season, the risk for RSV-associated hospital admissions was reduced by 83% in the immunised group compared with that in the non-immunised group (risk ratio [RR], 0.17; 95% CI, 0.11-0.26); this relative reduction in the risk persisted through the end of the second season.
• A sustained protective effect was observed for paediatric ICU admissions, with the immunised infants showing a 79% lower risk by the end of the second season (RR, 0.21; 95% CI, 0.11-0.48). Similar reductions were seen for emergency department visits and bronchiolitis recorded in primary care.
• RSV infections recorded in primary care by the end of the second season were lower in the immunised group; however, differences were attenuated and not significant.

IN PRACTICE:

"The absence of a delayed disease burden supports the hypothesis that nirsevimab reduces severe RSV-related outcomes during the first year of life without shifting the risk to subsequent seasons," the authors wrote.

SOURCE:

This study was led by Ermengol Coma, Primary Care Services Information System, Institut Català de La Salut, Barcelona, Spain. It was published online on September 13, 2025, in the European Journal of Pediatrics.

LIMITATIONS:

The validity of the findings may be affected by residual confounding from unmeasured variables. The non‑immunised group was small owing to the high uptake of nirsevimab. Additionally, this study excluded infants without primary care follow-up and those immunised outside the campaign, potentially omitting high‑risk infants with comorbidities.

DISCLOSURES:

This study did not report any funding source. One author reported receiving funding for lectures and attendance at international meetings and congresses from Sanofi, MSD, and Pfizer.

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