Immune persistence after 5 years of vaccination by Vi-DT vaccine among children below 2 years: Result from long-term observational study.

Typhoid fever remains a major public health threat to Low- and Middle-Income (LMIC) countries. The typhoid conjugate vaccine (TCV) is a promising vaccine platform to avert the disease as they elicit sufficient immune response in both adults and children. However, data on long-lasting immune response among children below 2 years are very limited. In this article we present the data collected from healthy Filipino children who were administered with Vi-DT vaccine. In this long-term follow-up study, we included children who had been vaccinated with Vi-DT vaccine and received a booster dose either in the short-term (24 weeks) or in the long-term (96 or 110 weeks). 285 participants divided into three cohort stratum, age strata 1 (6 months to less than 9 months), age strata 2 (9 months to 12 months), and age strata 3 (13 months to 23 months) from the phase II clinical trial were tracked and enrolled in the observational study. Blood samples were collected at 3rd, 4th and 5th year after the vaccination and compared descriptively against the comparator. We assessed the long-term immunogenicity using anti-Vi IgG and serum bactericidal antibody (SBA). Between 24 August 2021 to 28 July 2023, 268 healthy Filipino children were enrolled in the study who received at least one dose of Vi-DT or comparator vaccine from the Phase II clinical trial. Anti-Vi IgG seroconversion were higher in Vi-DT vaccine compared to the comparator, calculating 92.79 % (95 % CI: 88.44, 95.58), 88.68 % (83.71, 92.27), and 86.38 % (81.13,90.35) in the Vi-DT group, and 26.92 % (16.77, 40.25), 29.09 % (18.77, 42.14), and 25.45 % (15.81,38.30) in comparator group at 3rd, 4th and 5th year respectively. The seroconversion is higher in the Vi-DT long-term booster group than in short-term booster group in all age strata except age strata 3 at 5th year which is 83.78 % (68.86,92.35) and 88.89 % (74.69, 95.59) in Vi-DT long-term booster and short-term booster group, respectively. The SBA GMT response was observed to be higher in the Vi-DT vaccine group in all age strata compared to the comparator vaccine, calculating 932.06 (694.14, 1251.53), 958.61 (721.84, 1273.05) and 724.38 (537.89, 975.55) in the Vi-DT vaccine group and 657.63 (367.43, 1177.03), 796.27 (454.92, 1393.76), and 611.83 (338.10, 1107,18) in comparator group at 3rd year, 4th year and 5th year respectively. When administered as a long-term booster dose 2 years after the primary single-dose, the Vi-DT vaccine is capable of eliciting a sustained immune response for at least 5 years following vaccination. The result in this study will support the inclusion of Vi-DT vaccine in the routine immunization program in the endemic and semi-endemic areas.