This Phase Ⅱ/Ⅲ study is to evaluate whether or not there is a difference in time recovery of COVID-19 signs and symptoms through Day 29 between SIM0417/ritonavir and placebo.

The efficacy, safety, and tolerability of SIM0417/ritonavir compared to placebo will be investigated. The exposure to SIM0417 in this population will also be investigated.

Inclusion Criteria:

1. Participants ≥18 years of age (or the minimum country-specific age of consent if >18) at the time of signing the informed consent/assent form.
2. Initial positive SARS-CoV-2 tested by RT-PCR or rapid antigen test within 5 days (120 h) prior to the first dose of study drug collected from any respiratory tract specimen (e.g., oropharyngeal, NP or nasal swab, or saliva).
3. Initial onset of signs/symptoms attributable to COVID-19 within 3 days prior to the day of the first dose of study drug. The onset time of symptoms was defined as the time when body temperature first rose; or the onset of any of these COVID-19 symptoms.
4. At least one of the following symptoms of COVID-19 present within 24 hours prior to the first dose of study drug and meeting severity.
5. Has mild or moderate COVID-19.
6. Participants agree to take highly effective contraceptive measures from signing the informed consent to at least 1 month after the last dose of study intervention.
7. Participants who are willing and able to comply with all scheduled visits, treatment plan, laboratory tests, lifestyle considerations, and other study procedures.
8. Willing and able to provide written informed consent, or with a legal representative who can provide informed consent.

Exclusion Criteria:

1. Urgent or expected need for nasal high-flow oxygen therapy or positive pressure ventilation, invasive mechanical ventilation or Extracorporeal membrane oxygenation(ECMO).

2. Known medical history of active liver disease (other than nonalcoholic hepatic steatosis), including acute or chronic active hepatitis B or C infection, primary biliary cirrhosis, Child-Pugh Class B or C, or acute liver failure.

3. Receiving dialysis or have known moderate to severe renal impairment (ie, eGFR <45 mL/min/1.73 m2 within 6 months of the screening visit, using the serum creatinine-based CKD-EPI formula).

4. Compromised immune system (including patients receiving long-term immunosuppressant therapy, or those with progressed or relapsed cancer or human immunodeficiency virus [HIV] infection).

5. Moderate to severe congestive heart failure (New York Heart Association class III or IV) within 6 months prior to Screening, recent (within the past 6 months prior to Screening) cerebrovascular accident, myocardial infarction, coronary artery stenting, or uncontrolled hypertension (defined as documented systolic blood pressure > 160 mmHg or diastolic blood pressure > 100 mmHg).

6. Acute episode of chronic respiratory diseases, including bronchial asthma, chronic obstructive pulmonary disease.

7. Suspected or confirmed concurrent active systemic infection other than COVID-19 (eg, co-infected with influenza) that may interfere with the evaluation of response to the study intervention.

8. Any comorbidity requiring surgery within 14 days prior to study entry, or that is considered life-threatening within 30 days prior to study entry, as determined by the investigator.

9. Has hypersensitivity or other contraindication to any of the components of the study interventions.

10. Other medical or psychiatric conditions including recent (within the past year) or active suicidal ideation/behavior or laboratory abnormality that may increase the risk of study participation or, in the investigator's judgment, make the participant inappropriate for the study.

11. Clinical signs indicative of severe systemic illness with COVID-19, such as respiratory rate ≥30 per minute, heart rate ≥125 per minute, oxygen saturation (SpO2) of ≤93% on room air or the ratio of arterial oxygen partial pressure to fractional inspired oxygen (PaO2/FiO2) <300 obtained at rest within 24 hours prior to randomization.

12. Treatment with antivirals against SARS-CoV-2 within 14 days prior to the first dose of randomization.

13. Current or expected use of any medications or substances that are highly dependent on CYP3A4 for clearance and for which elevated plasma concentrations may be associated with serious and/or life-threatening events during study treatment and for 4 days after the last dose of study drug (refer to Appendix 6).

14. Concomitant use of any medications or substances that are strong inducers of CYP3A4 are prohibited within 21 days prior to randomization and during study treatment (refer to Appendix 6).

15. Has received (within the past 30 days or 5 × drug half-life prior to randomization, which is longer) or is expected to receive COVID-19 monoclonal antibody or convalescent COVID-19 plasma during study treatment.

16. Systemic glucocorticoid therapy (prednisone ≥ 20 mg/day or equivalent doses of other steroids) for at least 14 consecutive days within 30 days prior to randomization.

17. Has received any SARS-CoV-2 vaccine within 3 months prior to randomization.

18. Participating in another interventional clinical study with an investigational compound or device, including those for COVID-19.

19. Previous administration with any investigational drug or vaccine within 30 days (or as determined by the local requirement) or 5 half-lives (whichever is longer) prior to randomization.

20. Known prior participation in this trial or other trial involving SIM0417.

21. Women who are breastfeeding or have a positive pregnancy test in the pre-dose examinations. The following female patients who have documentation of either a or b below do not need to undergo a pregnancy test in the pre-dose examinations:

    1. Postmenopausal women (defined as cessation of regular menstrual periods for 12 months or more, and confirmed by a follicle-stimulating hormone test if <60 years old)
    2. Women who are surgically sterile by hysterectomy, bilateral oophorectomy, or tubal ligation

22. Has any condition for which, in the opinion of the investigator, participation would not be in the best interest of the participant or that could prevent, limit, or confound the protocol-specified assessments.

    For subjects who have conditional ECG at screening only:

23. Clinically relevant or significant electrocardiographic abnormalities (e.g., second-degree type II AV block, left bundle branch block, etc.), including electrocardiographic QT interval corrected for heart rate using Fridericia 's correction formula (QTcF = QT/(RR0.33)) > 450 ms (males) or > 470 ms (females).