This is an adaptive, randomized, double-blinded, placebo-controlled, Phase II/III study conducted to evaluate the effect of SCTA01 on participant survival and clinical efficacy in participants with severe COVID-19 admitted to high dependence or ICUs.

The study duration of subject participation will be up to: 120 days Participants will receive a single intravenous (IV) infusion of SCTA01 at Treatment day 1. Follow up visits will be up to 120 days or early withdrawal visit.

The study is a multicenter, adaptive, randomized, double-blinded, and placebo-controlled Phase II/III trial. It will be conducted globally. The study will evaluate the efficacy and safety of SCTA01 compared to placebo both given with BSC in participants with severe COVID-19. The subjects will be randomized by 1:1 ratio to SCTA01 and placebo group. The primary objective of the study is to evaluate participant survival from randomization to Day 29 between study group and control group. At the end of the Phase II part of the study, an interim analysis will be performed for safety run-in and futility stopping.

Inclusion Criteria:

• Male and female of ≥18years at time of enrollment;

• Subject (or legally authorized representative [LAR]) is able and willing to provide written or verbal informed consent, which includes compliance with study requirements and restrictions listed in the consent form.

• Female subjects must agree to use an approved highly effective birth control (BC) method (<1% failure rate per year) throughout the study (until completion of the Day 85 Follow-up Visit), unless documented to have a reproductive status of non-childbearing potential or is postmenopausal:

  1. Non-childbearing potential defined as pre-menopausal female with medical history of bilateral tubal ligation, bilateral oophorectomy (removal of the ovaries), or hysterectomy; hysteroscopic sterilization,
  2. Postmenopausal defined as 12 months of spontaneous amenorrhea

• Woman of childbearing potential (WCBP) who is already using an established method of highly effective contraception or agrees to use one of the allowed BC methods listed in the protocol, for at least 28 days prior to the start of dosing (as determined by the Investigator or designee) to sufficiently minimize the risk of pregnancy throughout study participation (until completion of the Day 90 Follow-up Visit).

• Hospitalized participants with severe COVID-19（6-8 point on WHO 10-Point Ordinal Scale):

  1. Point 6: Oxygen by NIV or high flow;
  2. Point 7: Intubation and MV, pO2/FiO2 ≥ 150 mmHg or SpO2/FiO2 ≥ 200 mmHg;
  3. Point 8: MV pO2/FiO2 < 150 mmHg (or SpO2/FiO2 < 200 mmHg) or vasopressors .

• Biological samples (not limited to any specific type) collected within 72 hours (allow retesting for potential subjects that tested positive beyond 72 hours) before randomization is laboratory-confirmed as SARS-CoV-2 infection (PCR, etc.);

• ≤ 14 days since the onset of COVID-19 symptoms.

Exclusion Criteria:

• Subject has been intubated for >72 hours. Note: in the event of extubation and re-intubation, the calculation for the number of hours the subject has been intubated begins at the first intubation
• Require or anticipated need for extracorporeal membrane oxygenation (ECMO) Suspected or proven septic shock or shock ;
• Alanine aminotransferase (ALT) or aspartate aminotransferase (AST) is >5 times higher than the upper limit normal range;
• Severe chronic respiratory disease (e.g., known chronic obstructive pulmonary disease [COPD], pulmonary arterial hypertension [PAH], idiopathic pulmonary fibrosis [IPF], interstitial lung disease [ILD]) requiring supplemental oxygen therapy or mechanical ventilation pre-hospitalization (e.g., prior to COVID-19 diagnosis)
• Use of prohibited medications
• Participants with severe COVID-19 who received convalescent plasma or COVID-19 vaccine , or anti-spike (S) SARS-CoV-2 therapy.
• Moribund condition in the opinion of the clinical team