Quantitative Methods for Evaluating Antibody Responses to Pneumococcal Vaccines: A Scoping Review

Streptococcus pneumoniae remains a leading cause of invasive diseases, particularly affecting young children and the elderly. Currently, two main types of pneumococcal vaccines are commercially available: polysaccharide vaccine (PPSV23) and conjugate vaccines (e.g., PCV20). Of over 100 identified pneumococcal serotypes, vaccines targeting 24 serotypes covered by PPSV23 and PCV20 (19 serotypes overlap between the two vaccines) have been developed, with serotype distribution varying by geography, age, and time. The immune response to pneumococcal vaccines differs across serotypes, vaccine types (polysaccharide vs. conjugate), and host factors. Quantitative methods for antibody assessment-particularly newer high-throughput assays-have emerged since 2000 to address limitations in conventional approaches. However, these methods have not been comprehensively reviewed. This scoping review aimed to systematically map the existing literature on quantitative methods used to assess antibody responses to pneumococcal vaccines. Specific objectives included the following: 1. summarizing conventional and novel quantitative immunoassays; 2. evaluating the current state of validation and application of these methods; 3. identifying knowledge gaps and methodological challenges. We followed the PRISMA-ScR guidelines. We included the following: 1. peer-reviewed, open-access papers related to immunoassays used for pneumococcal antibody assessment; 2. articles written in English; 3. Studies published between 2000 and 2023. We excluded the following: 4. studies focusing on other pathogens, employing different analytical methods, or using animal models. Articles meeting the eligibility criteria were primarily retrieved from PubMed and Scopus. If free full-text versions were unavailable there, Google Scholar or the original journal databases were consulted. All references were exported to EndNote 20 for further management. At the beginning of the review, a data-charting form was developed based on prior studies and commonly addressed themes. Additional charts were created to accommodate newly identified variables during the review. All charting tools were reviewed and finalized through discussion among all research team members. The included studies were classified into five thematic groups: 1. general descriptions of quantitative assessment methods, 2. assay development and validation, 3. comparative studies, 4. technical details of assay development, 5. interpretation of assay application findings. Of 1469 articles from PubMed and 2946 articles from Scopus initially identified, 55 articles met the inclusion criteria. The earliest methods included radioimmunoassays, later replaced by WHO-standardized ELISA. While ELISA remains the gold standard, it is limited by labor, cost, and throughput. Multiplex immunoassays (MIAs), including Luminex-based platforms, have demonstrated advantages in efficiency and scalability. However, many MIAs did not initially meet WHO validation criteria. More recent assays show an improved performance, yet interlaboratory variability and lack of standardized protective thresholds remain major limitations. This review provides the first comprehensive mapping of quantitative antibody assessment methods for pneumococcal vaccines. Although ELISA continues to serve as the benchmark, MIAs represent a promising next-generation approach. Continued efforts are needed to harmonize assay validation protocols and establish global standards for protective thresholds, which will enhance the reliability of vaccine efficacy monitoring across diverse populations.