Development and validation of immunological assays for preclinical evaluation of a novel bivalent typhoid conjugate vaccine

Currently there is no licensed bivalent typhoid conjugate vaccine against Salmonella Typhi (ST) and Salmonella Paratyphi A (SPT), which is a major public health concern causing enteric fever. Serum Institute of India Pvt. Ltd. (SIIPL) has developed a novel bivalent typhoid conjugate vaccine (BTCV), comprising of Vi polysaccharide conjugated to Tetanus Toxoid, (Vi-TT) and O-specific polysaccharide (OSP) conjugated to Diphtheria Toxoid (OSP-DT) conjugate arms. We in-house developed serological assays measuring quantitative Immunoglobulin G (IgG) by Enzyme-Linked Immunosorbent Assay (ELISA) and functional IgG by Serum Bactericidal Assay (SBA) to BTCV. The promising immune response to BTCV in pre-clinical models was verified using these methods. Further as per International Council for Harmonization (ICH) guidelines, we have standardized and validated, both IgG ELISA and SBA assessing the immunogenicity against S. Typhi and S. Paratyphi A. Both the assays were validated for parameters like Precision, Linearity/accuracy, robustness and specificity. Importantly, good immune response was observed for both arms of BTCV (Vi-TT and OSP-DT) when administered as a single dose regimen in the Good Laboratory Practice Toxicology, (GLP-TOX) studies, demonstrating non- interference response between the arms. Our study validates the utility of both assays in accurately detecting anti-Vi and anti-OSP specific immune responses. This underscores their value for testing clinical serum samples to assess typhoid-paratyphoid conjugate vaccine efficacy and can be applied to analyze human samples from upcoming clinical trials.