When a patient is due for both PPSV23 and a pneumococcal conjugate vaccine, why does it matter which one I give first?

When both a pneumococcal conjugate vaccine (PCV) and PPSV23 are to be administered, PCV is always recommended to be given before PPSV23, based on studies demonstrating a better response to serotypes common to both vaccines when PCV was given first. These vaccines should not be given at the same visit. The routine interval between PCV and PPSV23 is one year; however, the minimum interval between PCV and PPSV23 of 8 weeks may be used when protection from pneumococcal serotypes unique to PPSV23 is required quickly (e.g., due to a high-risk condition such as asplenia).