DYNAMICS AND ALTERNATION OF NEISSERIA MENINGITIDIS SEROGROUPS IN NIGER 2005—2024, FROM THE ERA OF POLYSACCHARIDE VACCINES TO THE ERA OF CONJUGATE VACCINES

OUSMANE Sani*, HAMADOU Idrissa, OUBAYYOU-YAROUKOY AMB, ALKASSOUM NB, BOUBACAR IB, INOUSSA Abdoulaye, MOUSSA SM, TAHIROU Zar, MAMANE KH, ABDOULKARIM BZ, SABO HS

Background: Niger, located in the heart of the African meningitis belt, is regularly hit by meningitis epidemics. Objective: To describe the spatiotemporal dynamics and alternation of Neisseria meningitidis serogroups over a 20-year period of surveillance. Method: national epidemiological and laboratory surveillance data recorded from 2005 to 2024 were collected and analyzed. Results: Prior to the introduction of MenAfriVac®, epidemic waves were mainly caused by Neisseria meningitidis serogroup A (NmA), which co-circulated with serogroups X and W. Reactive vaccination, done with polysaccharide vaccines (monovalent A, bivalent AC or tetravalent ACWY), only allowed to break the chain of transmission but did not offer lasting immunity. After the introduction of MenAfriVac® in 2010, a dramatic drop in NmA cases was observed. As of 2013, no case due NmA was detected in Niger. However, outbreaks due to serogroup W have been observed and the proportion of cases due to serogroups X and C, as well as Streptococcus pneumoniae, has increased. In 2015, serogroup C caused an unprecedented outbreak and was since continuing to dominate outbreaks in Niger. The country began using the new MEN5C vaccine for epidemic response and planned mass campaigns in December 2025. Like MenAfriVac, MEN5C is expected to eliminate outbreaks due to the 5 serogroups included. Nevertheless, vigilance must be the order of the day as nature has a horror of a vacuum. Conclusion: Niger's experience illustrates the success of targeted conjugate vaccination and the resulting capsular replacement phenomenon. Continuous surveillance is essential to monitor the evolution of serogroups and other bacterial etiologies with a view to eliminating meningitis by 2030.