What are the main differences between pneumococcal polysaccharide vaccine (PPSV23) and the pneumococcal conjugate vaccines (PCVs)?

The polysaccharide vaccine includes the different polysaccharides (chains of complex sugars) from different serotypes as the antigen. The conjugate vaccines have the polysaccharides for different serotypes attached (or conjugated) to a carrier protein. The immune response to the PPSV23 vaccine is a T-cell independent immune response, while the immune response to PCV vaccination is a T-cell dependent response that produces memory B-cells and reduces carriage of the bacteria in the respiratory track. The PPSV23 does not reduce bacterial carriage.