Abstract: The invention especially relates to multivalent vaccine compositions that can treat or prevent at least 60, preferably 75% of infections caused by Neisseria meningitidis especially of serogroup B. To this end, the invention in particular provides a lipooligosaccharide (LOS) of N. meningitidis in particular constituted by a lipid A, an inner core, an ? chain of L6 or L8 type, in which the heptose II residue of the inner core bears in position O-3 and in position O-6 or O-7 a phosphoethanolamine (PEA) substituent, and also to the construction of the strain of N. meningitidis that is capable of expressing such an LOS. The invention also relates to a strain of N. meningitidis of serogroup A that bears a lipooligosaccharide (LOS) in particular constituted by a lipid A, an inner core, an ? chain of L6 type, in which the heptose II residue of the inner core bears in position O-3 a phosphoethanolamine (PEA) substituent and does not bear a PEA substituent in positions O-6 and O-7.