Participants may be excluded from receiving one or more of the possible randomised options if their medical history suggests that a treatment may be contraindicated. Corticosteroids: none. Azithromycin: known long QT syndrome Convalescent plasma: known moderate or severe allergy to blood products REGN10933+REGN10987: IVIg use during admission for COVID-19 Aspirin: age <18 years, current use of aspirin or other antiplatelet therapy, recent bleeding that precludes use of aspirin in opinion of managing doctor Colchicine: Contraindications: Female participants ≥12 <50 years old (as contraindicated in women of child-bearing potential) Severe hepatic impairment (defined as requiring ongoing specialist care) Significant cytopaenia (e.g. neutrophil count <1.0 x109/L, platelet count <50 x109/L, reticulocyte count <20 x109/L [if available]) Concomitant use of strong CYP3A4 inhibitor (e.g. clarithromycin, erythromycin, systemic azole antifungal, HIV protease inhibitor) or P-gp inhibitor (e.g. ciclosporin, verapamil, quinidine). Cautions: dose frequency should be halved (i.e. 500 mcg once daily) in the following circumstances. If >1 of these is present, investigator should consider not including colchicine in randomisation. Concomitant use of moderate CYP3A4 inhibitor (e.g. diltiazem) Renal impairment: eGFR <30 mL/min/1.73m2 (either chronic or acute)

Participants may be excluded from receiving one or more of the possible randomised options if their medical history suggests that a treatment may be contraindicated. Corticosteroids: none. Azithromycin: known long QT syndrome Convalescent plasma: known moderate or severe allergy to blood products REGN10933+REGN10987: IVIg use during admission for COVID-19 Aspirin: age <18 years, current use of aspirin or other antiplatelet therapy, recent bleeding that precludes use of aspirin in opinion of managing doctor Colchicine: Contraindications: Female participants ≥12 <50 years old (as contraindicated in women of child-bearing potential) Severe hepatic impairment (defined as requiring ongoing specialist care) Significant cytopaenia (e.g. neutrophil count <1.0 x109/L, platelet count <50 x109/L, reticulocyte count <20 x109/L [if available]) Concomitant use of strong CYP3A4 inhibitor (e.g. clarithromycin, erythromycin, systemic azole antifungal, HIV protease inhibitor) or P-gp inhibitor (e.g. ciclosporin, verapamil, quinidine). Cautions: dose frequency should be halved (i.e. 500 mcg once daily) in the following circumstances. If >1 of these is present, investigator should consider not including colchicine in randomisation. Concomitant use of moderate CYP3A4 inhibitor (e.g. diltiazem) Renal impairment: eGFR <30 mL/min/1.73m2 (either chronic or acute)

Participants may be excluded from receiving one or more of the possible randomised options if their medical history suggests that a treatment may be contraindicated. Corticosteroids: none. Azithromycin: known long QT syndrome Convalescent plasma: known moderate or severe allergy to blood products REGN10933+REGN10987: IVIg use during admission for COVID-19 Aspirin: age <18 years, current use of aspirin or other antiplatelet therapy, recent bleeding that precludes use of aspirin in opinion of managing doctor

Participants may be excluded from receiving one or more of the possible randomised options if their medical history suggests that a treatment may be contraindicated. Corticosteroids: none. Azithromycin: known long QT syndrome Convalescent plasma: known moderate or severe allergy to blood products REGN10933+REGN10987: IVIg use during admission for COVID-19 Aspirin: age <18 years, current use of aspirin or other antiplatelet therapy, recent bleeding that precludes use of aspirin in opinion of managing doctor

Participants may be excluded from receiving one or more of the possible randomised options if their medical history suggests that a treatment may be contraindicated. Corticosteroids: none. Azithromycin: known long QT syndrome Convalescent plasma: known moderate or severe allergy to blood products REGN10933+REGN10987: IVIg use during admission for COVID-19

Participants may be excluded from receiving one or more of the possible randomised options if their medical history suggests that a treatment may be contraindicated. Corticosteroids: none. Azithromycin: known long QT syndrome Convalescent plasma: known moderate or severe allergy to blood products REGN10933+REGN10987: IVIg use during admission for COVID-19