ARM 4 1. Invasive mechanical ventilation or extracorporeal membrane oxygenation (ECMO) 2. History of pulmonary alveolar proteinosis (PAP). 3. Women of childbearing potential who are pregnant or breastfeeding. 4. Known hypersensitivity to lenzilumab or any of its components. 5 .Use of any FDA-approved anti-IL-6 therapy (eg. tocilizumab, sarilumab, siltukimab), anti-IL-1 therapy (eg. anakinra, canakinumab) or kinase inhibitor (eg.baracitinib, ibrutinib, acalabrutinib) therapy to treat COVID-19 within 8 weeks prior to randomization. Any live vaccine within 8 weeks prior to randomisation. Note that subjects receiving other FDA-approved immunomodulators to treat underlying autoimmune disorders such as rheumatoid arthritis, psoriasis, ankylosing spondylitis, asthma, chronic obstructive pulmonary disease, atopic dermatitis, multiple sclerosis, etc. would not be excluded. Participants on corticosteroids or dexamethasone are not excluded from the study. Note: Participants on convalescent plasma, remdesivir and/or hydroxychloroquine with or without azithromycin are not excluded from the study. 6. Use of GM-CSF agents (e.g., sargramostim) within 8 weeks prior to randomisation. 7. Expected survival < 24h in the opinion of the investigator. 8. Any condition that, in the opinion of the investigator, is likely to interfere with the safety and efficacy of the study treatment or puts the subject at unacceptably high risk from the study. 9. Participation in another interventional study of COVID-19

ARM 4 1. Invasive mechanical ventilation or extracorporeal membrane oxygenation (ECMO) 2. History of pulmonary alveolar proteinosis (PAP). 3. Women of childbearing potential who are pregnant or breastfeeding. 4. Known hypersensitivity to lenzilumab or any of its components. 5 .Use of any FDA-approved anti-IL-6 therapy (eg. tocilizumab, sarilumab, siltukimab), anti-IL-1 therapy (eg. anakinra, canakinumab) or kinase inhibitor (eg.baracitinib, ibrutinib, acalabrutinib) therapy to treat COVID-19 within 8 weeks prior to randomization. Any live vaccine within 8 weeks prior to randomisation. Note that subjects receiving other FDA-approved immunomodulators to treat underlying autoimmune disorders such as rheumatoid arthritis, psoriasis, ankylosing spondylitis, asthma, chronic obstructive pulmonary disease, atopic dermatitis, multiple sclerosis, etc. would not be excluded. Participants on corticosteroids or dexamethasone are not excluded from the study. Note: Participants on convalescent plasma, remdesivir and/or hydroxychloroquine with or without azithromycin are not excluded from the study. 6. Use of GM-CSF agents (e.g., sargramostim) within 8 weeks prior to randomisation. 7. Expected survival < 24h in the opinion of the investigator. 8. Any condition that, in the opinion of the investigator, is likely to interfere with the safety and efficacy of the study treatment or puts the subject at unacceptably high risk from the study. 9. Participation in another interventional study of COVID-19