Evaluate the Efficacy & Safety of Leronlimab in Patients With Severe or Critical COVID-19

Inclusion Criteria: 1. Male or female adult ≥ 18 years of age at time of screening. 2. Subjects hospitalized with severe or critical illness caused by coronavirus 2019 infection as defined below: A. Severe Illness: \- Diagnosed with COVID-19 by standard reverse transcriptase polymerase chain reaction (RT-PCR) assay or equivalent testing within 5 days of screening AND Symptoms of severe systemic illness/infection with COVID-19: \- At least 1 of the following: fever, cough, sore throat, malaise, headache, muscle pain, shortness of breath at rest or with exertion, confusion, or symptoms of severe lower respiratory symptoms including dyspnea at rest or respiratory distress AND Clinical signs indicative of severe systemic illness/infection with COVID-19, with at least 1 of the following: \- respiration rate (RR) ≥ 30, heart rate (HR) ≥ 125, saturated oxygen (SaO2) \<93% on room air or requires \> 2L oxygen by nasal canula (NC) in order maintain SaO2 ≥93%, PaO2/FiO2 \<300 (ratio of partial pressure of oxygen in arterial blood to fraction of inspired oxygen) AND \- None of the following: Respiratory failure (defined by endotracheal intubation and mechanical ventilation, oxygen delivered by high-flow nasal cannula, noninvasive positive pressure ventilation, or clinical diagnosis of respiratory failure in setting of resource limitations), Septic shock (defined by systolic blood pressure (SBP) \< 90 mm Hg, or Diastolic BP \< 60 mm Hg), Multiple organ dysfunction/failure B. Critical Illness: \- Diagnosed with COVID-19 by standard RT-PCR assay or equivalent testing within 5 days of screening AND Evidence of critical illness, defined by at least 1 of the following: \- Respiratory failure defined based on resource utilization requiring at least 1 of the following: Endotracheal intubation and mechanical ventilation, oxygen delivered by high-flow nasal cannula, noninvasive positive pressure ventilation, extracorporeal membrane oxygenation (ECMO), or clinical diagnosis of respiratory failure (in setting of resource limitation) OR \- Shock (defined by SBP \< 90 mm Hg, or Diastolic BP \< 60 mm Hg or requiring vasopressors) OR -Multiple organ dysfunction/failure 3. Subject, if intubated, positive end expiratory pressure (PEEP) \<15 cmH2O with PaO2/FiO2 \>150 mmHg. 4. Electrocardiogram (ECG) with no clinically significant findings as assessed by the Investigator 5. Subject (or legally authorized representative) provides written informed consent prior to initiation of any study procedures. 6. Understands and agrees to comply with planned study procedures. 7. Women of childbearing potential and their partner must agree to use at least one highly effective method of contraception (e.g., hormonal contraceptives \[implants, injectables, combination oral contraceptives, transdermal patches, or contraceptive rings\], intrauterine devices, bilateral tubal occlusion, or sexual abstinence) for the duration of the study. Exclusion Criteria: 1. Subjects with do-not-resuscitate (DNR) and/or do-not-intubate (DNI) orders or expected to be made DNR/DNI in setting of resource limitations or family wishes. 2. Not a candidate for dialysis or continuation of care (or full medical support) in setting of resource limitations. 3. Subject on continuous vasopressors (at the dose of norepinephrine \>20μg/min and/or vasopressin \>0.04 units/kg/min) for \>48 hours at time of screening. 4. Subjects who have a history of allergic reactions attributed to compounds of similar chemical or biologic composition to leronlimab (PRO 140) are not eligible. 5. Inability to provide informed consent or to comply with test requirements 6. Consideration by the investigator, for safety reasons, that the subject is an unsuitable candidate to receive study treatment 7. Pregnancy or breast feeding 8. Subject participating in another study with for an investigational treatment for COVID-19. Note: Subject who were prescribed (1) hydroxychloroquine or chloroquine with or without azithromycin, (2) Remdesivir, (3) convalescent plasma therapy, or (4) immunomodulatory treatments (including but not limited to sarilumab, clazakizumab, tocilizumab, and anakinra) for the off-label treatment of COVID-19 prior to study enrollment may be included and may continue to receive these agents as part of standard-of-care.