Rural-urban differences in oral typhoid vaccine responses in Uganda: contribution of Schistosoma mansoni.

Vaccine immunogenicity varies across populations, yet drivers remain unclear. We compared systemic (plasma) and mucosal (coprological) antibody responses to the oral typhoid vaccine Ty21a, quantified by ELISA, among Ugandan adolescents from urban and Schistosoma mansoni-endemic rural settings. Urban participants elicited higher responses. Causal mediation analysis indicated that S. mansoni partly accounted for reduced mucosal, but not systemic, responses in rural participants, highlighting implications for oral vaccine effectiveness in endemic settings.