The invention discloses a vaccine which comprises lipooligosaccharide (LOS) isolated from N. meningitidis and conjugated to a carrier protein. The invention also discloses a method of making the acellular vaccine. The method consists of two main steps. In the first step the lipooligosaccharide (LOS), chosen so it does not contain the lacto-N-neotetraose human antigen (LNnT), is detoxified by a novel procedure which uses hydrazine to remove the O-linked fatty acids. In the second step, the detoxified LOS (dLOS) is covalently conjugated to a carrier protein such as Tetanus Toxoid (TT).