Pneumococcal vaccine serotype-specific antibody levels among PCV-unvaccinated children in Karnataka, India.

The immunogenicity of pneumococcal vaccines is commonly assessed using post-vaccination serotype-specific IgG antibody concentrations. Therefore, characterization of baseline antibody levels in vaccine-naïve populations is essential for understanding naturally acquired immunity and accurately interpreting vaccine responses. However, data on baseline serotype-specific IgG levels in healthy, unvaccinated Indian children remain limited. This study aimed to measure baseline serotype-specific pneumococcal IgG antibody levels in healthy vaccine-naïve Indian children. Baseline serotype-specific IgG antibody concentrations against 24 pneumococcal vaccine serotypes were measured in 600 healthy PCV-unvaccinated Indian children aged ≤ 5 years using the WHO-recommended ELISA method. Geometric mean concentrations (GMCs) were calculated for each pneumococcal serotype, and variations in antibody levels were analyzed according to age, gender, and season. Baseline IgG GMCs varied widely across serotypes, ranging from 0.14 µg/mL for serotype 12 F to 5.65 µg/mL for serotype 14. Overall, 71% of serotypes exhibited GMCs above the proposed protective threshold of 0.35 µg/mL. Age-dependent increases in antibody concentrations were observed, with peak GMCs in children aged > 36-≤48 months. Gender-based differences were minimal, with significantly higher GMCs observed in females only for serotype 9 N, whereas seasonal variation was evident for selected serotypes. Healthy Indian children demonstrate considerable heterogeneity in baseline serotype-specific pneumococcal IgG levels prior to vaccination. These findings provide important baseline immunological data on serotype-specific pneumococcal IgG levels in healthy, PCV unvaccinated Indian children and may serve as a useful reference for interpreting future pneumococcal vaccine immunogenicity studies and seroepidemiological investigations.