The randomised, double-blind, placebo-controlled Phase II study will evaluate the efficacy and safety of MRx-4DP0004, in addition to standard-of-care in up to 90 patients hospitalised with symptoms indicative of COVID-19. Eligible participants will be randomised 2:1 to receive 14 days of treatment with either MRx-4DP0004 or placebo (two capsules twice daily). Participants will be monitored daily throughout the treatment period for improvement or progression of COVID-19 symptoms and adverse events. Following completion of the treatment period, participants will be followed-up at approximately 14 and 28 days post treatment. Patients will be recruited at sites across the UK.