How Gut Health Affects Immune Responses to the Oral Rotavirus Vaccine in Adults in Zambia

Inclusion Criteria: * Age ≥ 18 years and ≤ 50 years. * Able and willing to provide written informed consent. * Reside within Lusaka district and available for scheduled follow-up. * Willing to undergo two endoscopy procedures with serial sample collection. Exclusion Criteria: * Pregnant or breastfeeding. * Active gut disease requiring treatment (e.g., active peptic ulcer disease, gastrointestinal bleeding). * Known severe immunodeficiency (HIV with CD4 \< 200 cells/mm³, current cancer chemotherapy, systemic corticosteroids equivalent to \>20 mg/day prednisolone for \>2 weeks). * Severe comorbidities rendering endoscopy unsafe (e.g., severe cardiopulmonary disease, uncontrolled hypertension, oropharyngeal abnormalities). * Use of anticoagulants where suspension is unsafe or unwillingness to withhold anticoagulation when clinically indicated. * Receipt of any live vaccine within 30 days prior to enrolment or planned live vaccine within 30 days after Rotarix administration. * Any condition judged by the investigator to compromise participant safety or data integrity. * Participants who had a recent diarrhoea episode, or who have taken NSAID drugs or antibiotics, will be eligible for re-assessment after a month without this disqualifier. Pregnancy testing and counselling: Women of reproductive potential will require a negative urine pregnancy test within 24 hours prior to endoscopy and vaccination and will be counselled to avoid pregnancy during the first 30 days post-vaccination.