Study to Assess the Efficacy and Safety of STI-1558 in Adult Subjects With Mild or Moderate (COVID-19)

Inclusion Criteria: 1. Males or females aged ≥18 years; 2. Subjects comply with the diagnostic criteria for mild or moderate COVID-19 specified in the Protocol for the Diagnosis and Treatment of Novel Coronavirus Pneumonia (Trial Version 9) released by China's National Health Commission; 3. First presence of positive RT-PCR or rapid antigen assay for SARS-CoV-2 with nasopharyngeal or oropharyngeal swabs within 4 days prior to the first dose (≤4 days from the first dose); 4. First presence of at least 1 of 14 symptoms of COVID-19 within 3 days prior to the first dose (≤3 days/72 hours from the first dose) ; 5. Subjects with at least 2 of 11 target symptoms of COVID-19 within 24 hours prior to the first dose (≤24 hours from the first dose), including at least 1 designated symptom (Stuffy or runny nose, Sore throat or dry throat, Cough, Feeling hot or feverish, Shortness of breath or difficulty breathing), and score of at least 1 designated symptom ≥2 ; 6. Women of childbearing potential (WOCBP) (women of non-childbearing potential defined as women who have undergone hysterectomy or bilateral oophorectomy or bilateral salpingectomy or bilateral tubal ligation/closure, or who are infertile due to a congenital or acquired condition, or sexually mature women spontaneously menopausal for ≥ 12 months) must have a negative pregnancy test during the screening period; 7. Female subjects of childbearing potential or male subjects whose partners are of childbearing potential must agree to use a highly effective contraceptive method from screening to 30 days after the last dose; 8. Can understand and abide by the procedures and methods of this clinical trial. Exclusion Criteria: 1. Known allergy to any ingredient of the study drug; 2. Subjects comply with the diagnostic criteria for severe or critical COVID-19 specified in the Protocol for the Diagnosis and Treatment of Novel Coronavirus Pneumonia (Trial Version 9) released by China's National Health Commission prior to the first dose; 3. Subjects urgently require or is expected to require nasal high-flow oxygen therapy or non-invasive positive pressure ventilation, invasive mechanical ventilation or ECMO prior to the first dose; 4. Subjects with abnormal liver function at screening: total bilirubin ≥ 1.5 × upper limit of normal (ULN), alanine aminotransferase (ALT) or aspartate aminotransferase (AST) ≥ 3 × ULN; 5. Subjects with active or uncontrolled hepatopathy: including cirrhosis, Child-Pugh grade B or C, acute liver failure, etc.; 6. Subjects with concomitant severe renal insufficiency (estimated glomerular filtration rate \[eGFR\]\<45 mL/min, calculated using the serum creatinine based Chronic Kidney Disease Epidemiology Collaboration \[CKD-EPI\] equation, refer to Appendix 6 for the calculation formula) or on continuous renal replacement therapy, hemodialysis or peritoneal dialysis; 7. Subjects with impaired immune system (including subjects who are on systemic or inhaled corticosteroids or other immunosuppressants, or subjects with cancer progression or recurrence, or with human immunodeficiency virus \[HIV\] infection); 8. Subjects with acute episodes of chronic respiratory diseases (including bronchial asthma, chronic obstructive pulmonary disease, etc.); 9. Subjects with a history of dysphagia or any gastrointestinal disorder that affects drug adsorption; 10. Subjects with suspected or confirmed acute systemic infections other than COVID-19 (e.g., concomitant influenza), which may interfere with the evaluation of responses to the study intervention; 11. Subjects who have underwent major surgery within 14 days prior to the first dose, or have not completely recovered from surgery or plan to undergo surgery during the study period; 12. Subjects with complications that are, in the opinion of the investigator, life-threatening within 30 days prior to the first dose; 13. Subjects have received anti-SARS-CoV-2 chemical drugs such as 3CL protease inhibitors or RNA dependent RNA polymerase (RdRp) inhibitors or angiotensin converting enzyme 2 (ACE-2) and transmembrane protease serine 2 (TMPRSS2) degrading agents within 14 days prior to the first dose; 14. Subjects have received COVID-19 patients' convalescent plasma or COVID-19 human immunoglobulin or interleukin-6 (IL-6) inhibitors or anti-SARS CoV-2 neutralizing antibodies within 30 days or 5 half-lives (whichever is longer) prior to the first dose; 15. Subjects have had any SARS-CoV-2 vaccination within 3 months prior to the first dose; 16. Subjects have received any potent inhibitor or inducer of cytochrome P450 (CYP) 3A4 within 28 days or 5 half-lives (whichever is shorter) prior to the first dose; 17. Subjects who are currently participating in an interventional clinical study of other investigational drugs or devices, including other studies on COVID-19; 18. Subjects have received any investigational drug within 30 days or 5 half-lives (whichever is longer) prior to the first dose; 19. Subjects have previously participated in this study or other studies of STI-1558; 20. Pregnant or lactating women; 21. Subjects with active serious mental illness, medical disorder or other symptoms/conditions that, in the opinion of the investigator, may affect treatment, compliance or the ability to provide informed consent.