Why is it always recommended to give PCV15 before giving PPSV23?

If you stock PCV15 and plan to use the PCV15 and PPSV23 combined series to vaccinate patients for whom this is an option, you should always give the PCV15 first. PCV is always recommended to be given before PPSV23, based on studies demonstrating a better immune response to serotypes contained in both vaccines when the conjugate vaccine is given first. These vaccines should not be given at the same visit.

The routine interval between PCV15 and PPSV23 is one year; however, PPSV23 may be administered a minimum of 8 weeks after PCV15 when the recipient has an immunocompromising condition, cochlear implant, or cerebrospinal fluid leak. If PPSV23 is inadvertently administered first, wait a minimum of one year to administer PCV15, PCV20, or PCV21, because there is evidence that the immune response to serotypes contained in the PCV is diminished when PCV is administered soon after PPSV23.