Why is PCV15 vaccine recommended to be given first when a patient is getting a PCV15 and PPSV23 vaccine series?

PCV15 vaccine is recommended to be given first because this sequence provides the best immune response to both PCV15 and PPSV23 vaccine serotypes. An evaluation of immune response after a second pneumococcal vaccination administered 1 year after an initial dose showed that subjects who received PPSV23 as the initial dose had lower antibody responses to conjugate vaccine serotypes after subsequent administration of PCV13 than those who had received PCV13 as the initial dose followed by a dose of PPSV23. Lower antibody responses to conjugate vaccine serotypes were also seen in people 65 years and older who received PCV20 1 to 5 years after a dose of PPSV23 compared to those who received PCV20 at least 6 months after a dose of PCV13 or those who received PCV13 followed by PPSV23. PPSV23 is not recommended following administration of PCV20 or PCV21.