Exclusion Criteria Known severe allergic reactions to TCZ or other monoclonal antibodies Require continuous positive airway pressure (CPAP), bilevel positive airway pressure (BIPAP), or invasive mechanical ventilation Suspected active bacterial, fungal, viral, or other infection (besides COVID-19) In the opinion of the investigator, progression to death is imminent and inevitable within the next 24 hours, irrespective of the provision of treatments Immunocompromised (besides well-controlled HIV) or on immunosuppressive therapy (except for steroids for COVID), advanced cancer Have received oral anti-rejection or immunomodulatory drugs (including TCZ) within the past 3 months Participating in another interleukin (IL)-6 antagonist clinical trial or other drug clinical trials (participation in COVID-19 anti-viral trials may be permitted if approved by Medical Monitor) Alanine aminotransferase (ALT) or aspartate aminotransferase (AST) > 10 x upper limit of normal (ULN) detected within 24 hours at screening (according to local laboratory reference ranges) Absolute neutrophil count (ANC) < 1000/uL at screening (according to local laboratory reference ranges) Platelet count < 50,000/uL at screening (according to local laboratory reference ranges) Pregnant or breastfeeding, or positive pregnancy test in a pre-dose examination Treatment with an investigational drug within 5 half lives or 30 days (whichever is longer) of randomization (investigational COVID-19 antivirals may be permitted if approved by Medical Monitor) Any serious medical condition or abnormality of clinical laboratory tests that, in the investigator's judgment, precludes the patient's safe participation in and completion of the study Any history of Diverticulitis or GI perforation Use of systemic corticosteroids unless on a stable chronic dose

Exclusion Criteria Known severe allergic reactions to TCZ or other monoclonal antibodies Require continuous positive airway pressure (CPAP), bilevel positive airway pressure (BIPAP), or invasive mechanical ventilation Suspected active bacterial, fungal, viral, or other infection (besides COVID-19) In the opinion of the investigator, progression to death is imminent and inevitable within the next 24 hours, irrespective of the provision of treatments Immunocompromised (besides well-controlled HIV) or on immunosuppressive therapy (except for steroids for COVID), advanced cancer Have received oral anti-rejection or immunomodulatory drugs (including TCZ) within the past 3 months Participating in another interleukin (IL)-6 antagonist clinical trial or other drug clinical trials (participation in COVID-19 anti-viral trials may be permitted if approved by Medical Monitor) Alanine aminotransferase (ALT) or aspartate aminotransferase (AST) > 10 x upper limit of normal (ULN) detected within 24 hours at screening (according to local laboratory reference ranges) Absolute neutrophil count (ANC) < 1000/uL at screening (according to local laboratory reference ranges) Platelet count < 50,000/uL at screening (according to local laboratory reference ranges) Pregnant or breastfeeding, or positive pregnancy test in a pre-dose examination Treatment with an investigational drug within 5 half lives or 30 days (whichever is longer) of randomization (investigational COVID-19 antivirals may be permitted if approved by Medical Monitor) Any serious medical condition or abnormality of clinical laboratory tests that, in the investigator's judgment, precludes the patient's safe participation in and completion of the study Any history of Diverticulitis or GI perforation Use of systemic corticosteroids unless on a stable chronic dose