Immune responses to typhoid conjugate vaccine in a two dose schedule among Nepalese children <2 years of age.

Background: Previously, the Vi-typhoid conjugate vaccine (Vi-TT) was found to be highly efficacious in Nepalese children under 16 years of age. We assessed the immunogenicity of Vi-TT at 9 and 12 months of age and response to a booster dose at 15 months of age.
Methods: Infants were recruited at Patan Hospital, Kathmandu and received an initial dose of Vi-TT at 9 or 12 months of age with a booster dose at 15 months of age. Blood was taken at four timepoints, and antibody titres were measured using a commercial ELISA kit. The primary study outcome was seroconversion (4-fold rise in antibody titre) of IgG one month after both the doses.
Findings: Fifty children were recruited to each study group.Some visits were disrupted by the COVID19 pandemic and occurred out of protocol windows.Both the study groups attained 100 % IgG seroconversion after the initial dose. IgG seroconversion in the 9-month group was significantly higher than in the 12-month group (68.42 % vs 25.8 %, p < 0.001). Among individuals who attended visits per protocol, IgG seroconversion after the first dose occurred in 100 % of individuals (n = 27/27 in 9-month and n = 32/32 in 12-month group). However, seroconversion rates after the second dose were 80 % in the 9-month and 0 % in the shorter dose-interval 12-month group (p < 0.001) (n = 16/20 and n = 0/8, respectively).
Interpretation: Vi-TT is highly immunogenic at both 9 and 12 months of age. Stronger response to a booster in the 9-month group is likely due to the longer interval between doses.
Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: AJP reports financial support was provided by University of Oxford. AJP reports a relationship with Bill & Melinda Gates Foundation that includes: funding grants. AJP reports a relationship with National Institute for Health and Care Research that includes: funding grants. AJP reports a relationship with UKRI Medical Research Council that includes: funding grants. Andrew Pollard reports a relationship with Wellcome Trust that includes: funding grants. AJP is chair of the UK Department of Health and Social Care’s Joint Committee on Vaccination and Immunisation and was a member of WHOs SAGE until 2022. MV reports a relationship with Bill & Melinda Gates Foundation that includes: funding grants. MV is a contributor to intellectual property licensed by Oxford University Innovation to AstraZeneca. SK is a contributor to intellectual property licensed by Oxford University Innovation to AstraZeneca. All other authors declare that they have no conflict of interest.
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