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Cross-reactivity of pneumococcal capsular polysaccharides is a key element for formulating pneumococcal vaccines and evaluating vaccine efficacy. This study examined whether 23-valent pneumococcal polysaccharide vaccine (PPSV23), which only contains 6B, can elicit cross-functional immune responses against recently discovered serotypes (6C and 6D), as well as against 6A, in 2 adult age groups.Young adults (25–51 years; N = 28) and elderly subjects (over 65 years; N = 60) were immunized with PPSV23. Functional antibody responses were determined in pre- and postimmune sera via multiplexed opsonophagocytic killing assay against serotypes 6A/B/C/D.At postimmunization, the geometric mean opsonic indices (OIs) for 6B and nonvaccine serotypes (6A, 6C, and 6D) significantly increased in both age groups. The geometric fold increases of OIs for 6B/A/C/D significantly differed (18.2, 24.8, 3.1, and 7.1, respectively). Proportions of subjects with 4-fold increases in OIs for 6B/A/C/D were 73%, 70%, 31%, and 49%, respectively. Correlations of fold increases in OIs were highest between 6B and 6A, followed by 6B and 6D, then by 6B and 6C. Comparisons of young adults and the elderly revealed that most immunogenicity variables were higher in the former group.Our data demonstrated that 6B in PPSV23 induced cross-functional immune responses against serotypes 6A, 6C, and 6D, according to the degree of similarity in their capsular polysaccharide structures. In addition, we found significant age-related differences in PPSV23-induced cross-reactivity.