The invention particularly relates to multivalent vaccine compositions capable of treating or preventing at least 60% and preferably 75% of the infections caused by Neisseria meningitidis, in particular those of serogroup B. To this end, the invention substantially relates to lipooligosaccharide (LOS) of N. meningitidis particularly consisting of a lipid A, an inner core, and an L6 α chain, wherein the heptose II residue of the inner core has, at the O-3 position and at the O-6 or O-7 position, a phosphoethanolamine (PEA) substituent, as well as the structure of the N. meningitidis strain capable of expressing such LOS. The invention also relates to a strain of N. meningitidis of serogroup A that comprises lipooligosaccharide (LOS) particularly consisting of a lipid A, an inner core, and an L6 α chain, wherein the heptose II residue of the inner core has, at the O-3 position, a phosphoethanolamine (PEA) substituent and does not have any PEA substituent at the O-6 and O-7 positions. The abovementioned LOS or those from the mentioned strains can be used as vaccine antigens, in particular in multivalent, inter alia, bivalent, compositions in order to provide protection against major epidemiological complexes of N. meningitides, in particular of serogroup B.Linvention a notamment pour objet des compositions vaccinales multivalentes susceptibles de traiter ou de prévenir au moins 60 %, de préférence 75 % des infections dues à Neisseria meningitidis notamment de sérogroupe B. A cette fin, linvention fournit en particulier un lipooligosaccharide (LOS) de N. meningitidis constitué notamment dun lipide A, dun core interne, dune chaîne α de type L6, dans lequel le résidu heptose II du core interne porte en position O-3 et en position O-6 ou O-7 un substituant phosphoéthanolamine (PEA) ainsi que la construction de la souche de N. meningitidis capable dexprimer un tel LOS. Linvention porte également sur une souche de N. meningitidis de sérogroupe A qui possède un lipooligosacch
