This study is an adaptive Phase 2/3, randomized, double-blind, placebo-controlled trial to evaluate the efficacy and safety of sarilumab in hospitalized adults with severe or critical COVID-19. The study will be conducted in the United States (US) in up to 50 sites. Patients will be randomized in a 2:2:1 ratio to sarilumab 400 mg IV, 200 mg IV, or placebo in a stratified manner. Randomization will be stratified by severity of illness (severe, critical, multi-system organ dysfunction) and use of systemic corticosteroids for COVID-19