The invention relates to a conjugate of a saccharide covalently bound to a carrier protein, wherein the saccharide comprises repetitive units of 1,5 ribitol phosphate in which all the ribitol residues are substituted by N-acetyl D-glucosaminyl residues at the 4-position, and wherein said N-acetyl D-glucosaminyl residues are exclusively in anomeric configuration α or are exclusively in anomeric configuration β and wherein the carrier protein provides at least one epitope to the conjugate which is recognized by T helper lymphocytes.