Oral vaccines (OVs), provide protection against pathogens that infect mucosal surfaces and their potency relies on their capacity to elicit T- and B-cell responses directed to these surfaces. Oral vaccination efficacy has been found to vary considerably with differences in geographical locations and socioeconomic status. Specifically, in children living in resource-poor countries, undernourishment and chronic gastrointestinal (GI) infection are associated with the failure of OVs, which is a tragic outcome for the children who would benefit most from mucosal-based protection from infection. Both undernutrition and GI infection have been shown to profoundly affect the microbiota, inducing ‘dysbiosis’ characterized by narrowed bacterial diversity and increased frequency of bacterial clades associated with the induction of inflammation. Recent studies have demonstrated that the microbiota exerts a profound effect on the development of mucosal immune responses. Therefore, it seems likely that OV failure in resource-poor regions is affected by alterations to the immune response driven by dysbiotic changes to the microbiota. Here, we review the contribution of the microbiota to OV efficacy in the context of diet and GI infection.

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