Long-term Persistence of Immunogenicity After Primary Vaccination and Response to Booster Vaccination With Typhoid Conjugate Vaccine: Results of a Phase IV Extension Study.

OBJECTIVE: To evaluate the persistence of antibodies three years after primary vaccination with typhoid conjugate vaccine (TCV) of either Cadila Healthcare Ltd. (Cadila-TCV) or Bharat Biotech International Ltd. (Bharat-TCV) administered in a previous phase II/III study, and to study the booster dose response to Cadila-TCV. METHODS: This was an open-label, phase IV extension study conducted in tertiary care and multispecialty hospitals in India. 112 subjects (Cadila-TCV-57, Bharat-TCV-55) who had participated in previous study were enrolled. Of these, eligible subjects received a single-dose of Cadila-TCV and were followed-up for 28 days post-booster. Primary outcome was persistence of antibodies 3 years after primary vaccination and seroconversion (≥4-fold rise in antibody titre from baseline) 28 days post-booster. Safety was based on reported adverse events (AEs) post-booster. RESULTS: The baseline GMT reported in the current study was significantly higher than pre-vaccination GMT reported in the previous study. 89/112 (79.5%) subjects had antibody titer ≥10 IU/mL at baseline; eligible subjects (n=17) who had baseline antibody titre <10 IU/mL were administered booster dose. All the vaccinated subjects showed seroconversion post-booster. The GMTs reported at 10 days and 28 days post-booster were significantly higher as compared to GMTs reported after primary vaccination in previous study. 4 (23.5%) vaccinated subjects reported 9 AEs; all were solicited and of mild/moderate intensity. CONCLUSIONS: There was a significant persistence of immunogenicity after primary vaccination with both the TCVs, and robust immune response after booster vaccination with Cadila-TCV.