Introduction And Pharmacology

• Nirsevimab is a recombinant human IgG1-kappa monoclonal antibody engineered for the prevention of Respiratory Syncytial Virus (RSV) lower respiratory tract disease in infants.
• It incorporates YTE technology, representing a triple amino acid substitution in the Fc region to significantly increase binding affinity to the neonatal Fc receptor.
• This structural modification delays clearance and extends the serum half-life to approximately 60 to 100 days.
• A single intramuscular dose delivers sustained passive immunization for at least 5 months, providing continuous protection throughout an entire RSV season.

Mechanism Of Action

• Nirsevimab binds with high affinity to Antigenic Site Zero on the prefusion conformation of the RSV fusion (F) protein.
• By locking the viral F protein in its prefusion state, it physically blocks the conformational transition required for viral attachment and membrane fusion with host respiratory epithelial cells, thereby preventing viral entry and replication.

Clinical Indications And Guidelines

• The World Health Organization 2025 guidelines recommend nirsevimab for all infants to protect against severe RSV.
• The Indian Academy of Pediatrics Advisory Committee on Vaccines and Immunization Practices (ACVIP) 2025 schedule recommends it primarily for high-risk infants following parental consultation.
• High-risk conditions mandating prophylaxis include prematurity under 35 weeks, chronic lung disease, hemodynamically significant congenital heart disease, neuromuscular disorders, and severely immunocompromised states.
• The National Neonatology Forum 2025 Position Statement prioritizes high-risk babies in public settings but advises informing parents of healthy newborns for private use.
• It is indicated universally for infants under 8 months entering their first RSV season and for high-risk pediatric patients aged 8 to 19 months entering their second RSV season.

Dosage And Administration Protocol

• Nirsevimab is administered as a single intramuscular injection into the anterolateral aspect of the thigh.

Efficacy And Safety Profile

• Real-world data from the 2025-2026 seasons confirm 70% to 89% effectiveness against RSV-associated hospitalizations and an 80% effectiveness against intensive care unit admissions.
• It offers a major operational advantage over traditional palivizumab prophylaxis, which required strictly timed, monthly intramuscular injections and was restricted to highly select preterm cohorts.
• Nirsevimab provides immediate passive immunity without interfering with the active immune response generated by routine childhood vaccines.
• The safety profile remains excellent, with the most frequently reported adverse events being mild, self-limiting injection site reactions and transient pyrexia.
• Severe hypersensitivity or anaphylaxis to nirsevimab or its constituent excipients constitutes an absolute contraindication.

Disease Burden And Indian Context

• RSV remains a leading cause of hospitalization in children under 5 years, causing an estimated 3 to 5 million annual episodes in India.
• The Central Drugs Standard Control Organization granted marketing authorization for nirsevimab in August 2024, enabling private-sector availability.
• Its single-dose strategy substantially reduces the healthcare utilization burden and complements existing neonatal frameworks like Kangaroo Mother Care and Family Integrated Care in resource-limited settings.