Endothelial injury as a consequence of SARS-CoV-2 infection leads to a dysregulated host inflammatory response and activation of coagulation pathways. Macro- and micro-vascular thrombosis may contribute to morbidity, organ failure, and death. Therapeutic anticoagulation with heparin may improve clinical outcomes in patients with COVID-19 through anti-thrombotic, anti-inflammatory, and anti-viral activities of heparins. This pragmatic, Bayesian adaptive randomized controlled trial will determine whether therapeutic anticoagulation with heparin (subcutaneous low molecular weight heparin or intravenous unfractionated heparin) versus usual care reduces the need for intubation or death in hospitalized patients with COVID-19. The trial uses an adaptive design which was chosen to overcome limitations in available data to inform a priori estimation of event rates and possible effect sizes. The adaptive design also includes response-adaptive randomization based on baseline D-dimer level, probing for differential efficacy across subgroups defined based on initial D-dimer level. This Bayesian adaptive randomized trial will stop at a conclusion 1) when the posterior probability that the proportional odds ratio is greater than 1.0 reaches 99% (definition of benefit); 2) when the posterior probability that the proportional odds ratio is greater than 1.2 is less than 10% (definition of futility) or; 3) when the posterior probability that the proportional odds ratio is less than 1.0 is greater than 90% (definition of harm). The trial will enroll a maximum of 3,000 patients, although in many simulations the trial may require fewer patients. The trial is strategically aligned with the international REMAP-CAP/COVID platform trial to accelerate evidence generation.

This is a prospective, open-label, multicentre, Bayesian adaptive randomized clinical trial to establish whether therapeutic-dose parenteral anticoagulation improves outcomes for patients hospitalized with COVID-19 (e.g., reduces intubation or mortality). Participants will be randomized either to the investigational arm (therapeutic anticoagulation with heparin for 14 days or until "recovery" [defined as hospital discharge or liberation from supplemental oxygen if initially required], whichever comes first), or to the control arm (usual care, including thromboprophylactic dose anticoagulation according to local practice).

Inclusion Criteria:

1. Patients ≥18 years of age providing (possibly through a substitute decision maker) informed consent who require hospitalization anticipated to last ≥72 hours, for microbiologically-confirmed COVID-19, enrolled < 72 hours of hospital admission or of COVID-19 confirmation

• If the patient is already hospitalized and the COVID-19 diagnosis is due to an outbreak or an incidental finding, then enrollment can occur within 72 hours of a clinical syndrome attributable to COVID-19 that requires continued hospitalization (e.g. new or worsening oxygen requirements or acute kidney injury) which is further anticipated to extend the hospital admission by an additional 72 hours from randomization.

Exclusion Criteria:

1. Patients admitted to an ICU AND receiving organ support (i.e. high flow nasal oxygen, receiving non-invasive or invasive mechanical ventilation, or are requiring vasopressor/inotrope)

2. Patients for whom the intent is to not use pharmacologic thromboprophylaxis

3. Active bleeding

4. Risk factors for bleeding, including:

   1. intracranial surgery or stroke within 3 months;
   2. history of intracerebral arteriovenous malformation;
   3. cerebral aneurysm or mass lesions of the central nervous system;
   4. intracranial malignancy
   5. history of intracranial bleeding
   6. history of bleeding diatheses (e.g., hemophilia)
   7. history of gastrointestinal bleeding within previous 3 months
   8. thrombolysis within the previous 7 days
   9. presence of an epidural or spinal catheter
   10. recent major surgery <14 days
   11. uncontrolled hypertension (sBP >200 mmHg, dBP >120 mmHg)
   12. other physician-perceived contraindications to anticoagulation

5. Platelet count <50 x10^9/L, INR >2.0, or baseline aPTT >50 (if available per SOC testing)

6. Hemoglobin <80 g/L (to minimize the likelihood of requiring red blood cell transfusion if potential bleeding were to occur)

7. Acute or subacute bacterial endocarditis

8. History of heparin induced thrombocytopenia (HIT) or other heparin allergy including hypersensitivity

9. Current use of dual antiplatelet therapy

10. Patients with an independent indication for therapeutic anticoagulation

11. Patients in whom imminent demise is anticipated and there is no commitment to active ongoing intervention

12. Anticipated transfer to another hospital that is not a study site within 72 hours

13. Enrollment in other trials related to anticoagulation or antiplatelet therapy