Neisseria meningitidis is classified into 13 clinically significant serogroups based on the antigenic structure of the polysaccharide capsule. Six serogroups, A, B, C, Y, W135 and X, are responsible for virtually all cases of the meningococcal diseases in humans. Groups C, W-135 and Y have been commonly observed in developed countries within North America, Europe and Asia, and thus vaccines for these serogroups have been widely developed and commercialized for decades. Vaccine development for serogroup A meningococcus, the main cause of meningitis in African countries, however, has only recently been initiated, with prequalification of type A conjugate vaccine granted in June 2010.