a) To evaluate safety of convalescent plasma therapy in patients with severe Covid-19.b) To evaluate efficacy of convalescent plasma therapy with standard treatment versus standard treatment alone with respect to time to clinical improvement (TTCI) [Time Frame: up to 28 days].TTCI is defined as the time (in days) from initiation of study treatment (active or standard care) until a decline of two categories from admission status on a six-category ordinal scale of Clinical status which ranges from 1 (discharged) to 6 (death)(35).Six-category ordinal scale:6. Death;5. ICU; requiring IMV;4. ICU/hospitalization; requiring NIV/ HFNC therapy;3. Hospitalization; requiring supplemental oxygen (but not NIV/ HFNC);2. Hospitalization; not requiring supplemental oxygen;1. Hospital discharge.Abbreviation: IMV; invasive mechanical ventilation; NIV; non-invasive mechanical ventilation; HFNC; High-flow nasal cannula.

a) To evaluate safety of convalescent plasma therapy in patients with severe Covid-19.b) To evaluate efficacy of convalescent plasma therapy with standard treatment versus standard treatment alone with respect to time to clinical improvement (TTCI) [Time Frame: up to 28 days].TTCI is defined as the time (in days) from initiation of study treatment (active or standard care) until a decline of two categories from admission status on a six-category ordinal scale of Clinical status which ranges from 1 (discharged) to 6 (death)(35).Six-category ordinal scale:6. Death;5. ICU; requiring IMV;4. ICU/hospitalization; requiring NIV/ HFNC therapy;3. Hospitalization; requiring supplemental oxygen (but not NIV/ HFNC);2. Hospitalization; not requiring supplemental oxygen;1. Hospital discharge.Abbreviation: IMV; invasive mechanical ventilation; NIV; non-invasive mechanical ventilation; HFNC; High-flow nasal cannula.