Standard vs High Prophylactic Doses or Anticoagulation in Patients With High Risk of Thrombosis Admitted With COVID-19 Pneumonia (PROTHROMCOVID)

Inclusion Criteria: 1. Patients admitted to hospital with COVID-19, PCR and/or Antigens Test + SARS-CoV-2 infection or (presence of infiltrate compatible with Chest X-ray or TC) 2. Patients with, at least, one of the following evolution disease risk criteria: * Sat 02\<94% * Need for oxygen therapy or pAO2/FiO2\<300mmHg or estimated PaO2/FiO2 based on SpO2/FiO2\<300 mmHg. * DD\>1000µg/L * PCR \>150mg/L * IL6 \>40pg/ml 3. Age \> 18 years 4. Weight 50-100 Kg 5. After receiving oral and written information about the study, patient must give Informed Consent duly signed and dated before performing any activity related to the study. Exclusion Criteria: 1. Patients who need mechanical ventilation (invasive or non-invasive), high flow nasal cannula or admission to ICU at the moment of randomization. 2. Current diagnosis of acute bronchial asthma attack. 3. History or clinical suspicion of pulmonary fibrosis. 4. Current diagnosis or suspicion of pulmonary thromboembolism or deep vein thrombosis. 5. Patients who need anticoagulant treatment due to previous venous or arterial thrombotic disease, or due to atrial fibrillation. 6. Patients with pneumonectomy or lobectomy. 7. Renal failure with Glomerular filtration \<30 ml/min/1.73m2 8. Patients with contraindication for anticoagulant treatment. 9. Congenital bleeding disorders. 10. Hypersensitivity to tinzaparin or UFH or some of its excipients. 11. History of heparin-induced thrombocytopenia. 12. Active bleeding or situation that predispose to bleeding. 13. Moderate or severe anaemia (Hb\<10 g/dl) 14. Low platelet count \< 80000/µl 15. Patients with life expectancy less than 3 months due to primary disease evaluated by the physician. 16. Patients currently intubated or intubated between the screening and the randomization. 17. Pregnancy.