Post-Marketing Surveillance of Nirsevimab: Safety Profile and Adverse Event Analysis from Spain's 2023-2024 RSV Immunisation Campaign.

Respiratory syncytial virus (RSV) poses a significant health burden in children, being the major cause of lower respiratory tract infection (LRTI), including bronchiolitis. During the 2023-2024 RSV season, Spain introduced nirsevimab, a monoclonal antibody for universal RSV prophylaxis in infants. This study reviews the safety of nirsevimab through post-marketing surveillance. A descriptive pharmacovigilance study was made based on spontaneous reporting data of suspected adverse events (SAEs) from the Spanish Pharmacovigilance System for Medicinal Products for Human Use (SEFV-H) and industry reports. SAEs reported between September 2023 and May 2024 were extracted from the Spanish Pharmacovigilance Adverse Reactions Data (FEDRA) database. Cases were analysed by sex, age, severity, and SAEs classification using the Preferred Terms (PT) level of the Medical Dictionary for Regulatory Activities (MedDRA). Reporting rates were estimated based on immunization coverage and birth data. Sixty-seven cases reported 141 SAEs, yielding an overall rate of 23.1 cases per 100,000 doses. Common events included rash (8.51%), drug ineffectiveness (7.09%), and pyrexia (7.09%). Serious events constituted 53.70% of reports, including two fatalities (3.00%). No new safety signals or unexpected risks, such as antibody-dependent enhancement (ADE), were identified. SAEs reported peaked early in the campaign, reflecting heightened reporting in new immunization programs. The safety profile aligns with clinical trial findings and regulatory expectations, confirming nirsevimab's benefit-risk balance. Continued pharmacovigilance is critical for maintaining public trust in RSV prophylaxis. Nirsevimab demonstrated a favorable safety profile during Spain's initial universal RSV immunization campaign in infants, supporting its continued use in reducing RSV-related morbidity.