Methods for preparing an oligosaccharide-protein carrier immunogenic conjugate or a polysaccharide-protein carrier immunogenic conjugate. The methods include obtaining an oligosaccharide or polysaccharide having a KDO moiety located at the terminal reducing end of the oligosaccharide or polysaccharide that includes a carbonyl functional group; and reacting the carbonyl functional group of the KDO moiety with an aminooxylated protein carrier molecule resulting in a conjugate that includes a covalent oxime bond between the oligosaccharide and the protein carrier or the polysaccharide and the protein carrier.