A Study on Longevity of Immune Response after Vaccination with Salmonella Typhi Vi Conjugate Vaccine (Pedatyph™) in Children.

BACKGROUND AND OBJECTIVES Owing to the limitations of the conventional polysaccharide vaccines, global efforts have been made to develop conjugated polysaccharide vaccines for typhoid. Duration of immune response induced by these vaccines is critical to define the efficacy and frequency of required booster dose. This study was done to determine the duration of immune response following vaccination with Salmonella Typhi Vi conjugate vaccine (Pedatyph™) in children and to assess the booster effect of second dose of conjugate typhoid vaccine. MATERIALS AND METHODS Forty children were recalled from a cohort of 400 children, who received one dose or two doses of PedaTyph™, 30 months after vaccination. Ten non-vaccinated children were also recalled. Their serum samples were assessed by ELISA for anti Vi antibody. RESULTS Significantly high titers of anti-Vi polysaccharide IgG antibodies were present in vaccinated children even after 30 months of vaccination as compared to non-vaccinated children. Geometric mean titers (GMT) with 95% confidence intervals were 14 (4.8-29.8), 17 (7.4-33) and 6.4 (0.8-12) μg/ml for single dose, two doses and control group respectively. The children in two doses group had higher antibody titers as compared to single dose group. However, the difference was not significant. INTERPRETATION AND CONCLUSION PedaTyph™ was found to induce long term immune response as evidenced by presence of significant anti-Vi polysaccharide antibodies after 30 months of vaccination. No significant advantage of two doses regimen over one dose was found. Hence one dose vaccination with PedaTyph™ is suggested.