Emergence of serogroup X meningococcal disease in Africa: Need for a vaccine

•Serogroup X N. meningitidis (MenX) is emerging as a cause of meningitis in Africa.•Outbreaks were seen in Niger, Uganda, Kenya, Togo and Burkina Faso in 2006–2010.•Currently, no vaccine against MenX is available for the African Meningitis Belt.•The MenX polysaccharide capsule is the most likely target for vaccine development.•An affordable vaccine covering at least serogroups A, W, and X is needed.

Neisseria meningitidis is responsible for the seasonal burden and recurrent epidemics of meningitis in an area of sub-Saharan Africa known as the meningitis belt. Historically, the majority of the cases in the meningitis belt are caused by serogroup A meningococci. Serogroup C meningococci were responsible for outbreaks in the meningitis belt in the 1980s, while serogroup W (formerly W-135) has emerged as a cause of epidemic meningitis since 2000. Serogroup X meningococci have previously been considered a rare cause of sporadic meningitis, but during 2006–2010, outbreaks of serogroup X meningitis occurred in Niger, Uganda, Kenya,Togo and Burkina Faso, the latter with at least 1300 cases of serogroup X meningitis among the 6732 reported annual cases. While serogroup X has not yet caused an epidemic wave of the scale of serogroup A in 1996–1997 or serogroup W in Burkina Faso during 2002, the existing reports suggest a similar seasonal hyperendemicity and capacity for localised epidemics. Serogroup X incidence appears to follow a pattern of highly localised clonal waves, and in affected districts, other meningococcal serogroups are usually absent from disease. Currently, no licensed vaccine is available against serogroup X meningococci. Following the introduction of a monovalent serogroup A conjugate vaccine (MenAfriVac®) in the meningitis belt and the upcoming introduction of pneumococcal conjugate vaccines, vaccine-based prevention of serogroup X may become a public health need. The serogroup X polysaccharide capsule is the most likely target for vaccine development, but recent data also indicate a potential role for protein-based vaccines. A multivalent vaccine, preferably formulated as a conjugate vaccine and covering at least serogroups A, W, and X is needed, and the efforts for vaccine development should be intensified.