Procedure: Tinzaparin 4500 UI/day SC until hospital discharge.

Procedure: Tinzaparin 100 UI/Kg/day SC until hospital discharge.

Procedure: Tinzaparin 175 UI/Kg/day SC until hospital discharge.

day subcutaneously • Follow-up phase 1 (Day 15 to day 30): The information is obtained at discharge and on day +14, + 28 on 30 days. The latter information is obtained from patients, from medical records or by telephone contact at the patient's home.

• Follow-up phase 2 (At 3 months: Information is obtained from patients, medical records, or by telephone contact at the patient's home.

Combined variable that includes outcomes 2, 3 and 4 detailed below (reduction of suspicion of systemic thrombotic symptomatic events and/or need for mechanical ventilation and/or death at day 30 after randomization).

Reduction of symptomatic thrombotic events: rate of venous thromboembolism confirmed by objective test.

Mechanical ventilation (invasive or non-invasive) free survival.

Overall survival at 30 days: number of deaths.

Safety of the different strategies of prophylaxis and anticoagulation: number of bleedings and adverse reactions in each group:

Evaluation of the following variables:

Incidence of major bleeding, defined as meeting any of these criteria:

a) fatal bleeding or bleeding that occurs in a critical area or organ (for example, intracranial, intraspinal, intraocular, retroperitoneal, intraarticular or pericardial, or intramuscular with compartment syndrome), b ) causes a drop in hemoglobin level of 20 g/L or more, or c) requires the transfusion of 2 or more units of whole blood or packed red blood cells.

Incidence of clinically relevant non-major bleeding: manifest, spontaneous or post-traumatic bleeding, which does not meet the criteria for major bleeding but which in the judgment of the investigator is relevant.

Incidence of clinically relevant bleeding: all major and non-major hemorrhages clinically relevant.

Incidence of adverse reactions.