Comparison of the immunogenicity and safety of two different brands of salmonella typhi Vi capsular polysaccharide vaccine

Background: The recent emergence of multi-drug-resistant Salmonella strains highlights the need for better preventive measures, including vaccination. Safeand immunologic vaccines have been developed based on purified Vi polysaccharide. Objective: To compare the immune response elicited by two different brands of Salmonella Vi capsular polysaccharide vaccine (ViCPS). Setting and Design: Double blind, randomized (3:1), controlled, parallel, phase III study was conducted at two centres in India to compare the safety and immunogenicity of TypbarTM, the investigational vaccine with an already marketed vaccine "X", in healthy subjects aged between 12 -25 years. Material and Methods: A sample size of 184 subjects was calculated. Subjects were randomly distributed in two groups, immunized with single dose of TypbarTM or Vaccine "X". Serum samples were taken before 7 days and 4 weeks after immunization for the determination of antibodies to Vi polysaccharide, by ELISA method. Safety was assessed by physical examination, laboratory parameters before and after vaccination and by monitoring adverse events. Statistics: The geometric mean antibody titre (GMT) 4 weeks after vaccination was compared from respective pre-vaccination values by Wilcoxon signed rank test. Geometric mean of antibody levels before and after immunization and the ratio between them (Mann-Whitney test), the Seroconversion rates (Z test of proportions) and the adverse events (Fisher's exact testand Chi square test), were compared between two groups. P value < 0.05 was considered statistically significant. P values and 95% confidence intervals were estimated in two-tailed fashion. Results: 153 subjects (TypbarTM =116 and Vaccine "X" =37) were studied. 71.6% (95% CI=63.4%-79.8%) and 75.7% (95% CI=64.9% - 89.5%) were the seroconversion rates with TypbarTM and vaccine "X" respectively. The GMT values for Vi antibodies induced after TypbarTM and vaccine "X" were 10.23 TypbarTM and 13.46 mg/mL respectively and these values showed high significance when compared to their respective pre-immunization GMT values (P