The goal of this clinical trial is to learn if troxerutin works to prevent thrombotic events in mild or severe COVID-19 patients. It will also learn about the safety of troxerutin. The main questions it aims to answer are:

• Does troxerutin lower the number of thrombotic events in participants?
• What medical problems do participants have when taking troxerutin? Researchers will compare troxerutin to a placebo (a look-alike substance that contains no drug) to see if troxerutin works to prevent thrombotic events in COVID-19 patients.

Participants will:

• Take troxerutin or a placebo every day for 7 days.
• Visit the clinic at the first, fourth, seventh and 28th days after enrollment for checkups and tests
• Keep a diary of their symptoms and the number of times of thrombotic events, bleeding events and type II HIT-related thrombocytopenia

Inclusion Criteria:

As long as the patient meets all of the following conditions, adult patients aged 18 and above are eligible for admission:

► COVID-19 patients, defined as: positive RT-PCR (upper respiratory tract or lower respiratory tract) for SARS-CoV-2.

Mild COVID-19 patients are defined as:

Symptomatic patients meeting the case definition for COVID-19 without evidence of hypoxia or pneumonia. Common symptoms include fever, cough, fatigue, anorexia, dyspnea, and myalgia. Other nonspecific symptoms include sore throat, nasal congestion, headache, diarrhea, nausea/vomiting, and loss of smell/taste.

Severe COVID-19 patients are defined as:

Adolescents or adults with clinical signs of pneumonia (i.e., fever, cough, dyspnea, tachypnea) plus one of the following:

1. Respiratory rate ≥30 breaths/min

2. Severe respiratory distress

3. Oxygen saturation (SpO₂) ≤90% in room air

4. Progressive deterioration of clinical symptoms with lung imaging showing significant progression of lesions (>50%) within 24 to 48 hours.

   • Written informed consent provided according to Chinese law (by the patient, legal guardian, or deferred consent in emergencies).

Exclusion Criteria:

Patients with any of the following conditions will be excluded from the study:

• Pregnant or lactating women.
• Postpartum (within 6 weeks).
• Extreme weight (100 kilograms).
• Clinical need for heparin therapy.
• Bleeding related to coagulation disorders, acute clinically significant bleeding, active gastrointestinal ulcers, or any organic lesions with high bleeding risk.
• Platelet count <50 x 10^9/L.
• Surgery within the last 15 days, or within 24 hours after spinal or epidural anesthesia.
• History of intracranial hemorrhage, large ischemic stroke, known intracranial malformation or tumor, acute infective endocarditis.
• Severe renal impairment (creatinine clearance <30 mL/min).
• Iodine allergy.
• Long-term use of oxygen supplementation.
• Moribund patients or those expected to die during the current hospitalization due to underlying disease.
• Patients deprived of freedom and those undergoing institutional psychiatric care.
• Ward of the state or under guardianship.
• Participation in other anticoagulant intervention studies.