“To date there have been 12 vitamin C and COVID-19 trials published, including five randomised controlled trials (RCTs) and seven retrospective cohort studies. The current level of evidence from the RCTs suggests that intravenous vitamin C intervention may improve oxygenation parameters, reduce inflammatory markers, decrease days in hospital and reduce mortality, particularly in the more severely ill patients. High doses of oral vitamin C supplementation may also improve the rate of recovery in less severe cases. No adverse events have been reported in published vitamin C clinical trials in COVID-19 patients.
https://www.mdpi.com/2075-1729/11/11/1166/htm
And Vitamin C in Critically Ill Patients: An Updated Systematic Review and Meta-Analysis
https://www.mdpi.com/2072-6643/13/10/3564