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) hemofiltration/dialysis, or high-dose (>0.15?g/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 Note: Participants with an organ transplant are therefore excluded (except patients with corneal transplants not requiring immunosuppression). 12. History of allergic reaction, including anaphylaxis to any previous treatment with an anti-GM-CSF therapy 13. Received COVID-19 convalescent plasma within 48 hours of randomization Note: Participants who have received COVID-19 convalescent plasma but continue to worsen in the 48 hours after infusion of the convalescent plasma, in the opinion of the investigator, will become eligible for the study 14. Currently receiving chronic oral corticosteroids for a non-COVID-19 related condition in a dose higher than prednisone 10 mg or equivalent per day 15. Treatment with an investigational drug within 30 days of randomization 16. Participating in other drug clinical trials, including for COVID-19 17. Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) >5x upper limit of normal (ULN) 18. Platelets <50,000/mm³ 19. Hemoglobin =9 g/dl 20. Absolute neutrophil count (ANC) <1.5 x 109/l (neutropenia = Grade 2) 21. Estimated GFR =30 ml/min/1.73m² 22. Pregnant or breastfeeding females

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) hemofiltration/dialysis, or high-dose (>0.15?g/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 Note: Participants with an organ transplant are therefore excluded (except patients with corneal transplants not requiring immunosuppression). 12. History of allergic reaction, including anaphylaxis to any previous treatment with an anti-GM-CSF therapy 13. Received COVID-19 convalescent plasma within 48 hours of randomization Note: Participants who have received COVID-19 convalescent plasma but continue to worsen in the 48 hours after infusion of the convalescent plasma, in the opinion of the investigator, will become eligible for the study 14. Currently receiving chronic oral corticosteroids for a non-COVID-19 related condition in a dose higher than prednisone 10 mg or equivalent per day 15. Treatment with an investigational drug within 30 days of randomization 16. Participating in other drug clinical trials, including for COVID-19 17. Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) >5x upper limit of normal (ULN) 18. Platelets <50,000/mm³ 19. Hemoglobin =9 g/dl 20. Absolute neutrophil count (ANC) <1.5 x 109/l (neutropenia = Grade 2) 21. Estimated GFR =30 ml/min/1.73m² 22. Pregnant or breastfeeding females