DISCUSSION
IgA, typhoid
DISEASE, TYPHOID
TCV safety, immunogenicity, and efficacy have been demonstrated in adults and children as young as 6 months of age in typhoid-endemic areas [These data demonstrate robust longer-term immunogenicity in children who received coadministered TCV at the 15-month vaccination visit. The presence of comparatively low anti-Vi IgG antibody titers at baseline suggests a lack of environmental exposure to typhoid before 15 months of age and probably the disappearance of pre-existing maternal antibodies that could protect against disease. Increased anti-Vi IgG and anti-Vi IgA antibodies at 30–35 months post-vaccination in TCV recipients supports TCV’s broad and durable immune response for at least 2.5 years. These data support findings from India where GMTs in children 6–23 months vaccinated with TCV were 21-fold, 8-fold, and 5-fold above baseline at 3-, 5-, and 7-years post-vaccination and seroconversion rates were 72% and 44%, at 5- and 7-years post-vaccination, respectively [TCV recipients in Burkina Faso, vaccinated at the 15-month immunization visit, reported at least 94% seroconversion 28 days after vaccination in a previous analysis of anti-Vi IgG antibody [In the absence of an ideal assay for functional antibody assessment and its correlation with vaccine efficacy, anti-Vi IgG and IgA antibody estimates are accepted measures of TCV immunogenicity [A single dose of TCV is highly immunogenic in Burkinabe children vaccinated at the 15-month visit, providing both robust anti-Vi IgA and IgG responses at 28 days and durable immunity for 30–35 months after vaccination. Sustained immunogenicity for over 2.5 years after vaccination at 15 months informs vaccination schedule planning in endemic areas and suggests a booster dose, if needed, can likely wait until school entry. Future studies should continue to assess the longevity of vaccine-induced responses and the potential need for booster dosing in target populations.