Enoxaparin at Prophylactic or Therapeutic Doses in COVID-19

Patients with COVID-19 are at high risk of developing a venous thromboembolism (VTE) and it is essential that effective thromboprophylaxis with parenteral drugs (LMWH, UFH) is considered for all patients admitted to hospital especially in case of severe pneumonia. The aim of the study is the evaluation of efficacy and safety of enoxaparin at prophylactic dose (standard treatment) as compared to enoxaparin at therapeutic dose (OFF-LABEL treatment) in 300 COVID-19 infected patients with moderate-severe respiratory failure (PaO2/FiO2\<250) and/or increased D-dimer levels. After the admission to different Units (Infectious disease, Internal Medicine, Emergency Medicine, Pneumology), enoxaparin at prophylactic dose (standard of care) will be prescribed to all patients. The randomization of the single patient will be made when the the inclusion criteria (PaO2/FiO2 \<250 and/or D-dimer \>2000 ng/) will be satisfied. Patients with increased bleeding risk will be excluded (exclusion criteria). Patients will be divided into two arms: * arm A: enoxaparin at prophylactic dose (standard 4.000 IU; 6000 UI if body weight\>100 kg) * arm B: enoxaparin at therapeutic dose (70 U/Kg b.i.d. every 12 h) In both arms, enoxaparin treatment will be monitored clinically and with first and second line laboratory tests Venous compression ultrasound (CUS) will be performed at admission and after 7 days in case of a first negative exam and elevated D-Dimer levels, to rule out deep vein thrombosis. Enoxaparin at prophylactic dose (4000 IU) will be maintained in all patients for 4 weeks after discharge.