TOPLINE:

The administration of nirsevimab was associated with protection against respiratory syncytial virus (RSV) infection for up to 12 months in children younger than 24 months, with reductions in the odds of infection during the first 6 months, which were sustained through months 6-11. However, no significant protective effect was observed 12 months or beyond.

METHODOLOGY:

• Researchers conducted a retrospective analysis of real-world data to assess the long-term preventive effects of nirsevimab — a long-acting monoclonal antibody — against RSV infection in children younger than 24 months.
• Participants included 4607 children (45.8% girls) who received at least one dose of nirsevimab within 6 months before RSV testing, 860 children (46.4% girls) who received it 6-11 months before testing, and 532 children (46.2% girls) who received it at 12 months or later before testing; they were compared with a propensity score-matched cohort of 210,626 children who did not receive nirsevimab.
• The outcome of interest was RSV infection, defined as a positive result on a microbiologic test.

TAKEAWAY:

• The rate of RSV infection in the nirsevimab group was 8.5%, 8.0%, and 20.7% when nirsevimab was administered within 6 months, at 6-11 months, and at least 12 months before testing, respectively; the corresponding rates were 16.6%, 17.7%, and 17.3% in children who did not receive nirsevimab.
• Receiving nirsevimab within 6 months before testing was linked to a 53% lower likelihood of RSV infection than not receiving it, and a similar benefit was seen when it was given 6-11 months before testing (odds ratio, 0.41; P < .001 for both).
• No significant protective effect was observed for recipients who received nirsevimab 12 months or later before testing, indicating that this effect persisted for up to 12 months after administration.

IN PRACTICE:

“The results of this study may stimulate discussions regarding repeated dosing schedules for infants and young children,” the authors wrote.

SOURCE:

This study was led by Taito Kitano, DrPH, Nara Prefecture General Medical Center, Nara, Japan. It was published online on November 06, 2025, in Journal of Infection.

LIMITATIONS:

The real-world data used in this study may have had incomplete documentation of nirsevimab administration, potentially underestimating its preventive effect. The study did not evaluate history of maternal RSV vaccination. Additionally, the study could not assess the effect of nirsevimab on severe outcomes such as hospitalization, admission to critical care, and mortality.

DISCLOSURES:

This study was supported by the Japan Agency for Medical Research and Development and the Ministry of Health, Labour and Welfare’s Grant-in-Aid for the Promotion of Joint International Research. Some authors disclosed receiving grants and honoraria from the study funders, other organizations, and pharmaceutical companies.

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