First- and Second-Year Outcomes After Nirsevimab Immunization

Area of Science:

• Pediatrics
• Immunology
• Public Health

Background:

• Respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract infections (LRTI) in infants.
• Nirsevimab is a monoclonal antibody designed to prevent RSV-LRTI.
• The long-term effectiveness and broader clinical impact of nirsevimab require further investigation.

Purpose of the Study:

• To evaluate the sustained effectiveness of nirsevimab against RSV-LRTI hospitalizations across consecutive seasons.
• To assess nirsevimab's impact on non-RSV infection and asthma hospitalizations.
• To determine if nirsevimab's protective effects persist into the second year post-administration.

Main Methods:

• A retrospective cohort study utilizing the French National Health Data System.
• Matched 1:1 cohorts of nirsevimab-immunized and unimmunized infants born before the 2023 and 2024 immunization campaigns.
• Weighted Cox proportional hazards models were used to analyze RSV-LRTI, non-RSV infection, and asthma hospitalizations over one to two years of follow-up.

Main Results:

• Nirsevimab demonstrated significant effectiveness in reducing RSV-LRTI hospitalizations during the first year (66% in 2023, 72% in 2024).
• An increased rate of hospitalization for ear, nose, and throat (ENT) bacterial infections was observed in nirsevimab recipients.
• No protective association for RSV-LRTI was found in the second year, with increased hospitalizations for ENT and asthma noted.

Conclusions:

• Nirsevimab provides substantial protection against RSV-LRTI hospitalizations in infants during the first year of life.
• The protective effect of nirsevimab diminishes in the second year.
• Nirsevimab was associated with increased hospitalizations for ENT infections and asthma in the second year, warranting further study.