All Hib vaccines use polyribosylribitol phosphate (PRP) as the polysaccharide, but 4 different protein carriers are used in the 4 different Hib conjugate vaccines available: Diphtheria toxoid (PRP-D) Neisseria meningitidis outer membrane protein (PRP-OMP) Tetanus toxoid (PRP-T) Diphtheria mutant carrier protein CRM197 (HbOC) PRP-D and HbOC vaccines are no longer available in the US.