Hospitalised patients Male or female, aged 18 and above Clinically confident or proven Covid-19 disease* who require respiratory support** and who have not undergone significant de-escalation of respiratory support*** (i.e. are not in a recovery phase). Female participants of childbearing potential must be willing to use effective contraception for two weeks after final dose of IMP. Women will be advised to use a hormonal method, an intrauterine device (IUD) or intrauterine system (IUS), a barrier method or abstinence. *A clinically confident diagnosis is made where there is either swab positivity for Covid-19, or where the clinical presentation (including any of symptoms, clinical chemistry (e.g. raised D-dimer, raised CRP) and radiology (CXR, CT or ultrasound findings)) is consistent with likely Covid-19 infection. A pre-planned subgroup analysis will compare the primary outcome for those with swab positive and “clinically likely” disease. **At the time of recruitment, patients will be at least moderate oxygen therapy (>4 l/min O2 flow to mask or nasal cannulae, FiO2 > 0.3 for Venturi mask) to maintain pulse oximeter saturation, SpO2, in the target range set by the treating clinician. Other higher levels of oxygen support (including higher doses of oxygen, non-invasive respiratory support (continuous positive airway pressure (CPAP), high-flow nasal oxygen, or bi-level non-invasive positive pressure ventilation (NIPPV)) and invasive mechanical ventilation via an endotracheal tube can all be included, but patients are excluded if they have received >72 hours of invasive mechanical ventilation during their current illness. ***De-escalation of respiratory support is defined as a significant reduction in respiratory support that maintains saturation within the treating physicians’ target range within 24 hours of inclusion to this study. A significant reduction is any change in mode of oxygen delivery (i.e. intubated to non-invasive ventilation, non-invasive ventilation to standard oxygen therapy, or reduction of standard “wall” oxygen of more than 3 litres / min). Changes less than this will not be considered to be significant. Additional Inclusion Criteria for the Physiological Sub-Study Arterial line in place for clinical care Receiving either non-invasive respiratory support or invasive mechanical ventilation for which the inspired oxygen fraction can be measured.

Hospitalised patients Male or female, aged 18 and above Clinically confident or proven Covid-19 disease* who require respiratory support** and who have not undergone significant de-escalation of respiratory support*** (i.e. are not in a recovery phase). Female participants of childbearing potential must be willing to use effective contraception for two weeks after final dose of IMP. Women will be advised to use a hormonal method, an intrauterine device (IUD) or intrauterine system (IUS), a barrier method or abstinence. *A clinically confident diagnosis is made where there is either swab positivity for Covid-19, or where the clinical presentation (including any of symptoms, clinical chemistry (e.g. raised D-dimer, raised CRP) and radiology (CXR, CT or ultrasound findings)) is consistent with likely Covid-19 infection. A pre-planned subgroup analysis will compare the primary outcome for those with swab positive and “clinically likely” disease. **At the time of recruitment, patients will be at least moderate oxygen therapy (>4 l/min O2 flow to mask or nasal cannulae, FiO2 > 0.3 for Venturi mask) to maintain pulse oximeter saturation, SpO2, in the target range set by the treating clinician. Other higher levels of oxygen support (including higher doses of oxygen, non-invasive respiratory support (continuous positive airway pressure (CPAP), high-flow nasal oxygen, or bi-level non-invasive positive pressure ventilation (NIPPV)) and invasive mechanical ventilation via an endotracheal tube can all be included, but patients are excluded if they have received >72 hours of invasive mechanical ventilation during their current illness. ***De-escalation of respiratory support is defined as a significant reduction in respiratory support that maintains saturation within the treating physicians’ target range within 24 hours of inclusion to this study. A significant reduction is any change in mode of oxygen delivery (i.e. intubated to non-invasive ventilation, non-invasive ventilation to standard oxygen therapy, or reduction of standard “wall” oxygen of more than 3 litres / min). Changes less than this will not be considered to be significant. Additional Inclusion Criteria for the Physiological Sub-Study Arterial line in place for clinical care Receiving either non-invasive respiratory support or invasive mechanical ventilation for which the inspired oxygen fraction can be measured.