Abstract This supplement will support A5418. A5418 is Phase 3, randomized, placebo-controlled, double-blind trial of tecovirimat for the treatment of human monkeypox virus (HMPXV). The study will also include a cohort of people who will receive open-label tecovirimat including people with protocol-defined severe HMPXV, pregnant and breastfeeding individuals, individuals less than 18 years of age, people with severe immune suppression or skin conditions placing them at higher risk for severe disease. The primary endpoint is the time to clinical resolution defined as all skin lesions are either scabbed, desquamated or healed and all visible mucosal lesions healed.