Enhanced passive safety surveillance during the first use of the meningococcal vaccine A/C/W/Y/X conjugate for outbreak response in Niger and Nigeria

Abstract

Background: The African meningitis belt remains prone to recurrent invasive meningococcal disease (IMD) outbreaks, mainly from serogroups C, W, and X. In 2024, Niger and Nigeria conducted the first large-scale, real-world use of the pentavalent meningococcal conjugate vaccine (MMCV), covering Neisseria meningitidis serogroups A, C, W, Y, and X. This study describes enhanced passive safety surveillance implemented during these campaigns.

Methods: Enhanced passive safety surveillance was conducted during MMCV campaigns in Niger (May-July 2024) and Nigeria (March-June 2024). Surveillance included systematic adverse event reporting, digital tools (MedSafety App® in Nigeria, ODK Collect® in Niger), integrated supervision and systematic data monitoring. Adverse Events Following Immunization (AEFI) were analyzed by demographics, seriousness, and System Organ Class (SOC). Disproportionality analysis was performed using VigiBase® and Vigilyze® to identify potential safety signals.

Findings: A total of 4,881,027 individuals were vaccinated (Niger: 2,697,934; Nigeria: 2,183,093). There were 1109 AEFI reports (reporting rate: 22.7 per 100,000 vaccinees), with 11 (0.99%) classified as serious, including one death. Most AEFIs were mild and occurred on the day of vaccination; the most frequent were pyrexia, injection site reactions, myalgia, arthralgia, and headache. The majority of AEFI reports were among children aged 2-12 years (61.9%), followed by adolescents (17.4%) and young adults (15.2%).

Interpretation: The introduction of MMCV in Niger and Nigeria confirmed a robust safety profile in real-world outbreak settings, with very low rates of serious adverse events and no new safety signals detected. Enhanced passive surveillance, supported by digital tools and integrated supervision, enabled timely and comprehensive AEFI reporting. The observed safety outcomes were consistent with clinical trial data and previous vaccine introductions, reinforcing the reliability and generalizability of MMCV's safety. Sustained investment in pharmacovigilance infrastructure and ongoing training for healthcare workers remain essential for effective vaccine safety monitoring.

Keywords: Adverse events following immunization; Enhanced passive surveillance; Meningococcal vaccine; Pharmacovigilance; Reactive immunization; Vaccine safety.