Heterogeneous Serotype-Specific Immune Responses Following Sequential PCV13–PPSV23 Vaccination in Adults With Type 2 Diabetes Mellitus

Adults with type 2 diabetes mellitus (T2DM) are at increased risk of invasive pneumococcal disease due to impaired immune function; however, data on vaccine-induced immunity in this population remain limited. This study evaluated the immunogenicity of sequential pneumococcal vaccination with the 13-valent pneumococcal conjugate vaccine (PCV13) followed by the 23-valent pneumococcal polysaccharide vaccine (PPSV23) in Indian adults with T2DM. A total of 132 participants were enrolled, and serotype-specific IgG concentrations were measured before vaccination and eight weeks after PPSV23 administration. At baseline, over 70% of participants had IgG concentrations ≥1.3 μg/mL for serotypes 8, 9 N, 14, 15B, 19 A, 20, and 33F, while serotypes 3, 12F, 5, 4, and 17F had protective antibody levels in fewer than 50% of participants. Post-vaccination, IgG concentrations increased significantly for most serotypes. Serotype 14 demonstrated the highest geometric mean concentration (80.79 μg/mL), followed by serotypes 15B and 6B, while serotype 3 showed the lowest IgG concentration (0.94 μg/mL). Geometric mean fold rise ranged from 2.47 for serotype 3 to 54.61 for serotype 6B, with stronger responses in younger adults (35–54 years). Sequential pneumococcal vaccination was associated with increased serotype-specific IgG concentrations in adults with type 2 diabetes, demonstrating measurable serological immunogenicity.