Longevity of immune-response provided by at least one dose of vi-DT vaccine 5 years after vaccination among healthy Filipino adults and children

Typhoid Conjugate Vaccine (TCV) has been shown to elicit robust immune responses among adults and children. However, data on long-term immunogenicity are sparse. The International Vaccine Institute has conducted a Phase I clinical trial in The Philippines to investigate safety and immunogenicity among healthy individual 2-45 years old. In this study we present the results of a long-term immunogenicity study - 5 years after a typhoid conjugate vaccine composed of Vi polysaccharide conjugated to diphtheria toxoid (Vi-DT) and Vi-polysaccharide vaccine was administered. In this observational study, we followed up the participants who were vaccinated by either the Vi-DT vaccine or comparator vaccine, the Vi-polysaccharide vaccine, for five years after the enrollment in the phase I study. We assessed the safety and immunogenicity of the Vi-DT vaccine among healthy Filipino adults and children, aged 2-45 years and compared it descriptively to a Vi polysaccharide vaccine. Participants who had received at least one dose of either vaccine and agreed to provide at least one blood sample were enrolled to the study. Blood samples were collected to assess the longevity of immunity using anti-Vi IgG and serum bactericidal antibody (SBA) at three different time points: 3 years, 4 years, and 5 years after the vaccination. Between 30 October 2019 and 13 January 2022, blood samples were collected from 137 participants who had received at least one dose of Vi-DT or Vi polysaccharide vaccine, 3 years post-vaccination. Anti-Vi IgG seroconversion of Vi-DT vaccinees was higher in all age strata; 98.44 % (95 % confidence interval (CI): 91.67, 99.72) at year 3, 100.00 % (95 % CI: 94.08, 100.00) at year 4 and 98.41 % (95 % CI: 91.54, 99.72) at year 5. Anti-Vi IgG GMT was higher in the Vi-DT group among all age strata as well. Seroconversion by SBA in the Vi-DT group was higher in the 18-45 age group calculating 75.00 % (95 % CI: 50.50, 89.82), 86.67 % (95 % CI: 62.12, 96.26), and 60.00 % (95 % CI: 35.75, 80.18) at 3rd, 4th and 5th year post-vaccination, respectively. The Vi-DT vaccine provides immunity for at least five years after vaccination. However, data beyond 5 years would be valuable to inform the need for booster doses.