Purified polysaccharide vaccines work by activation of B cells in a T-independent manner, producing predominantly IgM and little memory B cells.1 Protein-polysaccharide conjugate vaccines (such as Prevnar/PCV7 which uses 7 prevalent polysaccharides to bind to non-toxic variant of diphtheria toxin, CRM197) allow the protein to present antigen on B cells and CD40/CD40L interaction, while T cells allow antibody class switching from IgM to IgG producing memory cells and longer response. Conjugated vaccines usually use polyribosylribitol phosphate (PRP) conjugated with protein carriers and conjugate vaccines for Haemophilus influenzae and Neisseria meningitidis (using outer membrane proteins, OMP) have already been developed.2
However, conjugate vaccines may not work in high-risk categories like HIV-positive children3 and asplenics,2 and the PPV23 vaccine failure comorbid elderly4needs to be identified and followed up. IgG subclass measurement for evaluation of vaccine response is vital; anti-IgG1 pneumococcal antibodies in children (both with normal and abnormal immunity) and anti-IgG2 antibodies in adults are the best discriminatory laboratory measure.5 The future seems certain for conjugate vaccines, and PPV23 may end up being a test to see polysaccharide response in an individual.