PPSV23 testable-serotype denominator (PCV-conjugate era)

How many PPSV23 serotypes remain interpretable as a polysaccharide challenge after a patient’s conjugate-vaccine (PCV13/15/20/21) exposures, and whether the ≥70%/≥50% response rule is still meaningful.

Evidence tier: Guideline-derived.

Formula / logic

Testable serotypes = PPSV23 serotypes NOT contained in any conjugate vaccine received. A serotype present in a received PCV reflects a T-dependent (conjugate) response and is not interpretable as a T-independent polysaccharide challenge. Apply the response rule (≥70% of measured serotypes for ≥6 y/adults; ≥50% for children 2-6 y) to the contracted denominator. Reference exposures (PPSV23-unique serotypes remaining): no PCV → 23; PCV13 → 11 (removes 1,3,4,5,6B,7F,9V,14,18C,19A,19F,23F); PCV15 → 9; PCV20 → 4 (2, 9N, 17F, 20); PCV20 + PCV21 → 1 (serotype 2 only, because PCV21/Capvaxive contains 9N, 17F and 20).

Interpretation

n ≥ 10: rule fully interpretable. 4 ≤ n < 10: contracted, interpret with caution (a single non-responder can flip the result). n < 4: the proportion rule loses discriminatory meaning, consider alternative assays (Salmonella typhi Vi polysaccharide response, opsonophagocytic activity).

Evidence & citations

  1. Zuzolo J, Zulfiqar MF, Spoelhof B, et al. Functional testing of humoral immunity in the Prevnar 20 era. Ann Allergy Asthma Immunol. 2025;134(3):279-283. PMID 39681261
  2. Orange JS, Chinen J, Horner CC, et al. 2025 Inborn errors of immunity practice parameter (JTFPP/AAAAI/ACAAI/CIS). Ann Allergy Asthma Immunol. 2026;136(4):426-493.e1. PMID 41936423
  3. Serotype compositions from FDA package inserts: Pneumovax 23 (PPSV23), Prevnar 13, Vaxneuvance (PCV15), Prevnar 20, Capvaxive (PCV21/V116). FDA package inserts

Clinician decision support. Verify against the cited source. Not a substitute for clinical judgment. 100% on-device; no patient data is stored or transmitted.