Key Inclusion Criteria:

- Laboratory-confirmed SARS-CoV-2 infection as determined by polymerase chain reaction
(PCR), result from any specimen (or other commercial or public health assay) within 2
weeks prior to randomization and no alternative explanation for current clinical
condition

- Hospitalized with illness of any duration with evidence of pneumonia, requires
supplemental oxygen and/or assisted ventilation and meets one of the following:

- Phase 2 and Phase 3 Cohort 1:

Meets 1 of the following criteria at baseline:

- Severe disease OR

- Critical disease OR

- Multi-system organ dysfunction OR

- Immunocompromised

- Phase 3 Cohort 2:

Patients must be receiving mechanical ventilation to treat respiratory failure due to
COVID-19

- Phase 3 Cohort 3:

Patients must be receiving supplemental oxygen to treat hypoxemia delivered by one of the
following devices:

- Non-rebreather mask, OR

- High-flow device with at least 50% FiO2, OR

- Non-invasive positive pressure ventilator

- Ability to provide informed consent signed by study patient or legally acceptable
representative

- Willingness and ability to comply with study-related procedures/assessments

Key Exclusion Criteria:

- In the opinion of the investigator, not expected to survive for more than 48 hours
from screening

- Presence of any of the following abnormal laboratory values at screening: absolute
neutrophil count (ANC) less than 2000 mm3, aspartate aminotransferase (AST) or alanine
aminotransferase (ALT) > 5 x upper limit of normal (ULN), platelets <50,000 per mm3

- Treatment with anti-IL 6, anti-IL-6R antagonists, or with Janus kinase inhibitors
(JAKi) in the past 30 days or plans to receive during the study period

- Current treatment with the simultaneous combination of leflunomide and methotrexate

- Known active tuberculosis (TB), history of incompletely treated TB, suspected or known
extrapulmonary TB, suspected or known systemic bacterial or fungal infections

- Participation in a double-blind clinical research study evaluating an investigational
product (IP) or therapy within 3 months and less than 5 half-lives of IP prior to the
screening visit (The use of remdesivir, hydroxychloroquine, or other treatments being
used for COVID-19 treatments in the context of an open-label study, Emergency Use
Authorization (EUA), compassionate use protocol or open-label use is permitted)

- Any physical examination findings, and/or history of any illness, concomitant
medications or recent live vaccines that, in the opinion of the study investigator,
might confound the results of the study or pose an additional risk to the patient by
their participation in the study

- Known systemic hypersensitivity to sarilumab or the excipients of the drug product

- Phase 3 Cohort 2 and Cohort 3 only:

- Known or suspected history of immunosuppression or immunodeficiency disorder

- Patients who require renal replacement therapy for acute kidney injury at
randomization or who required renal replacement therapy within 72 hours prior to
randomization

- Patients who have circulatory shock requiring vasopressors at randomization or within
24 hours prior to randomization

- Use of extracorporeal life support (eg, ECMO) or, in the opinion of the investigator,
there is a high likelihood that extracorporeal life support will be initiated within
48 hours after randomization

NOTE: Other protocol defined inclusion / exclusion criteria may apply