Part 1 1. Progression to death is imminent and inevitable within the next 48 hours, irrespective of the provision of treatments, in the opinion of the investigator. 2. Multiple organ failure according to the investigator’s judgement or a Sequential Organ Failure assessment (SOFA score) >10 if in the ICU. 3. Extracorporeal membrane oxygenation (ECMO) haemofiltration/dialysis, or high-dose (>0.15 μg/kg/min) noradrenaline (or equivalent) or more than one vasopressor. 4. Current serious or uncontrolled medical condition (e.g. significant pulmonary disease [such as severe COPD or pulmonary fibrosis], heart failure [NYHA class III or higher], significant renal dysfunction, acute myocardial infarction or acute cerebrovascular accident within the last 3 months) or abnormality of clinical laboratory tests that, in the investigator's judgment, precludes the participant's safe participation in and completion of the study. 5. Untreated systemic bacterial, fungal, viral, or other infection (other than SARS-CoV-2). 6. Known active tuberculosis (TB), history of untreated or incompletely treated active or latent TB, suspected or known extrapulmonary TB. 7. Known HIV regardless of immunological status. 8. Known HBsAg and/or anti-HCV positive. 9. Currently receiving radiotherapy, chemotherapy or immunotherapy for malignancy. 10. Received monoclonal antibody therapy (e.g. tocilizumab, sarilumab) within the past 3 months prior to randomization, including intravenous immunoglobulin or planned to be received during the study. 11. Received immunosuppressant therapy including but not limited to cyclosporin, azathioprine, tacrolimus, mycophenolate, JAK inhibitors (e.g. baricitinib, tofacitinib, upadacitinib) within the last 3 months prior to randomization or planned to be received during the study. 12. History of allergic reaction, including anaphylaxis to any previous treatment with an anti- GM-CSF therapy. 13. Currently receiving chronic oral corticosteroids for a non-COVID-19 related condition in a dose higher than prednisone 10 mg or equivalent per day. 14. Treatment with an investigational drug within 30 days of randomization. 15. Participating in other drug clinical trials, including for COVID-19. 16. Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) >5x upper limit of normal (ULN) 17. Platelets <50,000/mm3 18. Hemoglobin ≤9 g/L 19. Absolute neutrophil count (ANC) <1.5 x 109/L (neutropenia ≥ Grade 2) 20. Estimated GFR ≤30 mL/min/1.73m2 21. Pregnant or breastfeeding females. Part 2 1. Progression to death is imminent and inevitable within the next 48 hours, irrespective of the provision of treatments, in the opinion of the investigator. 2. Multiple organ failure according to the investigator’s judgement or a Sequential Organ Failure assessment (SOFA score) >10 if in the ICU. 3. Extracorporeal membrane oxygenation (ECMO) haemofiltration/dialysis, or more than one inotrope/vasopressor of any class. 4. Current serious or uncontrolled medical condition (e.g. significant pulmonary disease [such as severe COPD or pulmonary fibrosis], heart failure [NYHA class III or higher], severe renal dysfunction, acute myocardial infarction or acute cerebrovascular accident within the last 3 months), severe dementia, severe disability or abnormality of clinical laboratory tests that, in the investigator's judgment, precludes the participant's safe participation in and completion of the study. 5. Untreated systemic bacterial, fungal, viral, or other infection (other than SARS-CoV-2). 6. Known active tuberculosis (TB), history of untreated or incompletely treated active or latent TB, suspected or known extrapulmonary TB. 7. Known HIV regardless of immunological status. 8. Known HBsAg and/or anti-HCV positive. 9. Currently receiving radiotherapy, chemotherapy or immunotherapy for malignancy. 10. Received monoclonal antibody therapy (e.g. tocilizumab, sarilumab) within the past 3 months prior to randomization, including intravenous immunoglobulin or planned to be received during the study. 11. Received immunosuppressant therapy including but not limited to cyclosporin, azathioprine, tacrolimus, mycophenolate, JAK inhibitors (e.g. baricitinib, tofacitinib, upadacitinib), nintedanib, DMARDs (e.g. methotrexate) within the last 3 months prior to randomization or planned to be received during the study. History of allergic reaction, including anaphylaxis to any previous treatment with an anti- GM-CSF therapy. 12. Received COVID-19 convalescent plasma within 48 hours of randomization. 13. Currently receiving chronic oral corticosteroids for a non-COVID-19 related condition in a dose higher than prednisone 10 mg or equivalent per day. 14. Treatment with an investigational drug within 30 days of randomization. 15. Participating in other drug clinical trials, including for COVID-19. A complete list of Exclusion criteria for Part 2 can be found in the protocol Section 5.4

Part 1 1. Progression to death is imminent and inevitable within the next 48 hours, irrespective of the provision of treatments, in the opinion of the investigator. 2. Multiple organ failure according to the investigator’s judgement or a Sequential Organ Failure assessment (SOFA score) >10 if in the ICU. 3. Extracorporeal membrane oxygenation (ECMO) haemofiltration/dialysis, or high-dose (>0.15 μg/kg/min) noradrenaline (or equivalent) or more than one vasopressor. 4. Current serious or uncontrolled medical condition (e.g. significant pulmonary disease [such as severe COPD or pulmonary fibrosis], heart failure [NYHA class III or higher], significant renal dysfunction, acute myocardial infarction or acute cerebrovascular accident within the last 3 months) or abnormality of clinical laboratory tests that, in the investigator's judgment, precludes the participant's safe participation in and completion of the study. 5. Untreated systemic bacterial, fungal, viral, or other infection (other than SARS-CoV-2). 6. Known active tuberculosis (TB), history of untreated or incompletely treated active or latent TB, suspected or known extrapulmonary TB. 7. Known HIV regardless of immunological status. 8. Known HBsAg and/or anti-HCV positive. 9. Currently receiving radiotherapy, chemotherapy or immunotherapy for malignancy. 10. Received monoclonal antibody therapy (e.g. tocilizumab, sarilumab) within the past 3 months prior to randomization, including intravenous immunoglobulin or planned to be received during the study. 11. Received immunosuppressant therapy including but not limited to cyclosporin, azathioprine, tacrolimus, mycophenolate, JAK inhibitors (e.g. baricitinib, tofacitinib, upadacitinib) within the last 3 months prior to randomization or planned to be received during the study. 12. History of allergic reaction, including anaphylaxis to any previous treatment with an anti- GM-CSF therapy. 13. Currently receiving chronic oral corticosteroids for a non-COVID-19 related condition in a dose higher than prednisone 10 mg or equivalent per day. 14. Treatment with an investigational drug within 30 days of randomization. 15. Participating in other drug clinical trials, including for COVID-19. 16. Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) >5x upper limit of normal (ULN) 17. Platelets <50,000/mm3 18. Hemoglobin ≤9 g/L 19. Absolute neutrophil count (ANC) <1.5 x 109/L (neutropenia ≥ Grade 2) 20. Estimated GFR ≤30 mL/min/1.73m2 21. Pregnant or breastfeeding females. Part 2 1. Progression to death is imminent and inevitable within the next 48 hours, irrespective of the provision of treatments, in the opinion of the investigator. 2. Multiple organ failure according to the investigator’s judgement or a Sequential Organ Failure assessment (SOFA score) >10 if in the ICU. 3. Extracorporeal membrane oxygenation (ECMO) haemofiltration/dialysis, or more than one inotrope/vasopressor of any class. 4. Current serious or uncontrolled medical condition (e.g. significant pulmonary disease [such as severe COPD or pulmonary fibrosis], heart failure [NYHA class III or higher], severe renal dysfunction, acute myocardial infarction or acute cerebrovascular accident within the last 3 months), severe dementia, severe disability or abnormality of clinical laboratory tests that, in the investigator's judgment, precludes the participant's safe participation in and completion of the study. 5. Untreated systemic bacterial, fungal, viral, or other infection (other than SARS-CoV-2). 6. Known active tuberculosis (TB), history of untreated or incompletely treated active or latent TB, suspected or known extrapulmonary TB. 7. Known HIV regardless of immunological status. 8. Known HBsAg and/or anti-HCV positive. 9. Currently receiving radiotherapy, chemotherapy or immunotherapy for malignancy. 10. Received monoclonal antibody therapy (e.g. tocilizumab, sarilumab) within the past 3 months prior to randomization, including intravenous immunoglobulin or planned to be received during the study. 11. Received immunosuppressant therapy including but not limited to cyclosporin, azathioprine, tacrolimus, mycophenolate, JAK inhibitors (e.g. baricitinib, tofacitinib, upadacitinib), nintedanib, DMARDs (e.g. methotrexate) within the last 3 months prior to randomization or planned to be received during the study. History of allergic reaction, including anaphylaxis to any previous treatment with an anti- GM-CSF therapy. 12. Received COVID-19 convalescent plasma within 48 hours of randomization. 13. Currently receiving chronic oral corticosteroids for a non-COVID-19 related condition in a dose higher than prednisone 10 mg or equivalent per day. 14. Treatment with an investigational drug within 30 days of randomization. 15. Participating in other drug clinical trials, including for COVID-19. A complete list of Exclusion criteria for Part 2 can be found in the protocol Section 5.4