Background Oral Polio Vaccine (OPV) may improve resistance to non-polio-infections. We tested if OPV reduced the risk of illness and mortality prior to COVID-19-vaccines being available. Methods During the early COVID-19 pandemic, houses in urban Guinea-Bissau were randomised 1:1 to intervention or control. Residents aged 50 + years were invited to participate. Participants received bivalent OPV (single dose) or nothing. Rates of mortality, admissions, and consultation for infections (primary composite outcome) during 6 months of follow-up were compared in Cox proportional hazards models adjusted for age and residential area. Secondary outcomes included mortality, admissions, consultations, and symptoms of infection. Results We followed 3726 participants (OPV: 1580; Control: 2146) and registered 66 deaths, 97 admissions and 298 consultations for infections. OPV did not reduce the risk of the composite outcome overall, hazard ratio (HR) = 0.97 (95%CI: 0.79-1.18). OPV reduced the risk in males (HR = 0.71 (0.51-0.98) but not in females (HR = 1.18 (0.91-1.52), p for same effect = 0.02). OPV also reduced the risk in BCG scar-positive (HR = 0.70 (0.49-0.99) but not in scar-negative participants (HR = 1.13 (0.89-1.45), p = 0.03). OPV had no overall significant effect on mortality (HR = 0.96 (0.59-1.55)), admissions (HR = 0.76 (0.49-1.17)) or recorded consultations (HR = 0.99 (0.79-1.25)), but the OPV group reported more episodes with symptoms of infection (6050 episodes; HR = 1.10 (1.03-1.17)). Conclusion In line with previous studies, OPV had beneficial non-specific effects in males. Trial Registration www.clinicaltrials.gov/NCT04445428

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