Acute respiratory distress syndrome and multiple organ failure in patients with Coronavirus disease 2019 (severe and critical illness) are primarily due to widespread inflammation. 

Vitamin C administration has proven to improve the capillary circulation, microvascular barrier function and arteriolar reactivity caused by vasoconstrictors. It is also a tissue antioxidant that can effectively remove oxygen free radicals produced by myocardial tissues, macrophages and ischemia-reperfusion tissues. 

Thus they act as powerful antioxidant and can also inhibit vascular inflammatory exudation.

A study reported that the risk of 28-day mortality is reduced and Oxygen support status is improved (without adverse events) by administering the high-dose vitamin C (loading dose of 6g intravenous infusion/12 hr on the first day, and 6g/day for the following 4 days) contrasting the standard therapy.

Source: Ageing (Albany NY). 2021 Feb 26;13(5):7020-7034. doi: 10.18632/aging.202557. Epub 2021 Feb 26. PMID: 33638944; PMCID: PMC7993712.