Integrated analysis of humoral and T-cell responses to pneumococcal vaccination in allogeneic hematopoietic stem cell transplant recipients.

Introduction: Allogeneic hematopoietic stem cell transplant (HSCT) recipients remain highly susceptible to pneumococcal infection despite current vaccination strategies, and the contribution of T-cell-mediated immunity to protection in this population is not fully defined. Methods: We conducted a prospective study evaluating humoral and cellular immune responses to sequential pneumococcal vaccination with the 13-valent conjugate vaccine (PCV13) followed by the 23-valent polysaccharide vaccine (PPV23) in allogeneic HSCT recipients. Immune responses were assessed through serotype-specific antibody quantification, CD4 Results: Conjugate vaccination induced antigen-specific CD4 Discussion: This study provides, to our knowledge, the first integrated analysis of both humoral and T-cell immune responses to pneumococcal vaccination in allogeneic HSCT recipients, offering a translational perspective that links immunological mechanisms with clinical relevance. Our findings indicate that conjugate vaccination is essential for priming both cellular and humoral immunity, whereas polysaccharide boosting primarily broadens serotype coverage but may attenuate previously established immune responses. In the context of emerging higher-valency conjugate vaccines, including the recently introduced 21-valent formulation incorporating novel serotypes, these results support a reassessment of the need for polysaccharide boosters and inform optimisation of pneumococcal vaccination strategies in immunocompromised hosts at high risk for invasive disease.