The overall purpose of this study is to evaluate whether tecovirimat is an efficient and safe antiviral in the treatment of monkeypox in adults and adolescents (14 years old and older). The primary objective is to evaluate the clinical efficacy, as assessed by time to all visible lesion(s) resolution, of tecovirimat treatment + Standard of Care (SOC) compared to placebo + SOC for patients with monkeypox. The secondary objective is to evaluate the clinical efficacy, as assessed by mortality, hospitalization, complications, duration of symptoms and virological shedding, and the safety of tecovirimat treatment + SOC compared to placebo + SOC in patients with monkeypox.