The main aim is to determine whether vitamin C can reduce 28-day all-cause mortality or persistent organ dysfunction compared with placebo in patients with severe and critical ill COVID-19 patients.

Participants will randomly receive HIVC or placebo for 4 days once enrolled. The primary outcome is a composite of death or persistent organ dysfunction (defined as dependency on vasopressors, mechanical ventilation, or CRRT) at day 28 after randomization.

Inclusion Criteria:

1. Adults (age 18 years or older).
2. Diagnosed with COVID-19 according to the Diagnosis and Clinical Management of COVID-19. (trial version 10).
3. severe and critical ill patients with COVID-19.
4. Patients who voluntarily participate in the study and sign the informed consent form.

Exclusion Criteria:

1. Patients with a history of allergy to VC.
2. Pregnant or lactating women.
3. Patients with end-stage malignant tumour.
4. Patients with an expected survival duration of less than 24 hours.
5. Patients with cerebral hernia and severe craniocerebral injury.
6. Patients with diabetes.
7. Patients with a previous history of G-6-PD deficiency.