Characterization of Unusual Serogroups of Neisseria meningitidis.

Most cases of invasive meningococcal disease (IMD) in Europe are caused by isolates of the Neisseria meningitidis serogroups B, C, W, and Y. We aimed to explore cases caused by other unusual serogroups. We retrospectively screened IMD cases in the databases of the National Reference Center for Meningococci and Haemophilus influnezae in France between 2014 and 2023. Age, sex, serogroups, and genetic lineage distributions were analyzed. We also measured complement deposition on the bacterial surface and tested coverage by vaccines against serogroup B. Cases due to isolates of serogroups other than B, C, W, and Y represented 1.6% of all 3610 IMD cases during the study period with 59 cases and a median age of 21.5 years of age. The corresponding isolates were non-groupable (26 cases), serogroup X (21 cases), serogroup E (11 cases), and one isolate belonged to serogroup Z. Only a low proportion (7.4%) belonged to the hyperinvasive genetic lineages. Isolates of serogroup E bound a significantly higher amount of complement on their surface and were mainly detected in patients with terminal complement pathway deficiencies. Isolates of these unusual serogroups were shown to be covered by vaccines licensed against meningococci B. Surveillance of these isolates needs to be enhanced.