1. onset of covid-19 symptom fulfilling inclusion criterion #5 \>14 days prior to randomization 2. hospitalized with hypoxemia (as defined in inclusion #4) for \>72 hours prior to randomization (the 72-hour window for randomization begins when the patient first meets the hypoxemia inclusion criteria after hospital admission) 3. pregnancy 4. breastfeeding 5. prisoners 6. end-stage renal disease (esrd) on dialysis 7. patient undergoing comfort care measures only such that treatment focuses on end-of- life symptom management over prolongation of life. 8. the treating clinician expects inability to participate in study procedures or participation would not be in the best interests of the patient 9. known allergy/hypersensitivity to imp or its excipients fostamatinib arm-specific

1. onset of covid-19 symptom fulfilling inclusion criterion #5 \>14 days prior to randomization 2. hospitalized with hypoxemia (as defined in inclusion #4) for \>72 hours prior to randomization (the 72-hour window for randomization begins when the patient first meets the hypoxemia inclusion criteria after hospital admission) 3. pregnancy 4. breastfeeding 5. prisoners 6. end-stage renal disease (esrd) on dialysis 7. patient undergoing comfort care measures only such that treatment focuses on end-of- life symptom management over prolongation of life. 8. the treating clinician expects inability to participate in study procedures or participation would not be in the best interests of the patient 9. known allergy/hypersensitivity to imp or its excipients fostamatinib arm-specific

onset of covid-19 symptom fulfilling inclusion criterion #5 >14 days prior to randomization hospitalized with hypoxemia (as defined in inclusion #4) for >72 hours prior to randomization (the 72-hour window for randomization begins when the patient first meets the hypoxemia inclusion criteria after hospital admission) pregnancy breastfeeding prisoners end-stage renal disease (esrd) on dialysis patient undergoing comfort care measures only such that treatment focuses on end-of- life symptom management over prolongation of life. the treating clinician expects inability to participate in study procedures or participation would not be in the best interests of the patient known allergy/hypersensitivity to imp or its excipients fostamatinib arm-specific

onset of covid-19 symptom fulfilling inclusion criterion #5 >14 days prior to randomization hospitalized with hypoxemia (as defined in inclusion #4) for >72 hours prior to randomization (the 72-hour window for randomization begins when the patient first meets the hypoxemia inclusion criteria after hospital admission) pregnancy breastfeeding prisoners end-stage renal disease (esrd) on dialysis patient undergoing comfort care measures only such that treatment focuses on end-of- life symptom management over prolongation of life. the treating clinician expects inability to participate in study procedures or participation would not be in the best interests of the patient known allergy/hypersensitivity to imp or its excipients fostamatinib arm-specific

onset of covid-19 symptom fulfilling inclusion criterion #5 >14 days prior to randomization hospitalized with hypoxemia (as defined in inclusion #4) for >72 hours prior to randomization (the 72-hour window for randomization begins when the patient first meets the hypoxemia inclusion criteria after hospital admission) pregnancy breastfeeding prisoners end-stage renal disease (esrd) on dialysis patient undergoing comfort care measures only such that treatment focuses on end-of- life symptom management over prolongation of life. the treating clinician expects inability to participate in study procedures or participation would not be in the best interests of the patient known allergy/hypersensitivity to imp or its excipients

onset of covid-19 symptom fulfilling inclusion criterion #5 >14 days prior to randomization hospitalized with hypoxemia (as defined in inclusion #4) for >72 hours prior to randomization (the 72-hour window for randomization begins when the patient first meets the hypoxemia inclusion criteria after hospital admission) pregnancy breastfeeding prisoners end-stage renal disease (esrd) on dialysis patient undergoing comfort care measures only such that treatment focuses on end-of- life symptom management over prolongation of life. the treating clinician expects inability to participate in study procedures or participation would not be in the best interests of the patient known allergy/hypersensitivity to imp or its excipients