Table 2.

PICO of all included studies (N = 6).

S. No.	Author and Year	Population/Patient (P)	Intervention (I)	Comparator (C)	Outcome (O)	Standard Treatment
1	Zhang J et al., 2021 [14]	Patients with COVID pneumonia, having/at risk of multiple organs injury, P/F ratio <300 mmHg and admitted in the ICU, adults (age ≥18 and < 80 years)	Within 48 h after admission to the ICU high-dose intravenous vitamin C (24g/day): 12 g of vitamin C (diluted in 50 ml) BD for 7 days at a rate of 12 ml/h + standard therapy	Placebo: Bacteriostatic water for injection (same volume)+ Standard therapy	Primary: Invasive mechanical ventilation-free days in 28 days (IMVFD28). Secondary: (i)28-day mortality; (ii) organ failure [SOFA score]; (iii) inflammatory markers (IL-6, TLC, absolute neutrophil & lymphocyte counts, procalcitonin and CRP	Oseltamivir and azithromycin; LMWH; Piperacillin/tazobactan; hydrocortisone (1 mg/kg/day); Lung protective ventilation if IMV needed.
2	Kumari P et al., 2020 [15]	SARS-CoV-2 patients	50 mg/kg/day of IV Vitamin C + standard therapy	Standard therapy	Treatment duration, hospital stay, need for invasive ventilation, mortality	Antipyretics, dexamethasone, and prophylactic antibiotics
3	Siahkali S et al., 2021 [19]	>18 year patients with confirmed (RTPCR based) or suspected COVID-19 [based on clinico-radiological pattern e.g. fever, dyspnea, dry cough and/or CT finding suggestive of COVID] and SpO2<93% at admission or >48h from the first COVID-19 treatment.	Vitamin C (1.5 g every 6 h, total 6 g daily)+standard therapy	Standard therapy alone	Primary: reduction in mortality, duration of hospital stay, and need for ICU admission. Secondary: Improvements in vitals (e.g. SpO2), clinical parameters	Oral lopinavir/ritonavir (400/100 mg) BD and single dose of oral hydroxychloroquine (400 mg) on the first day of hospitalization.
4	Hakamifard A et al., 2021 [18]	Adult patients with COVID based on lab (RT-PCR) test and/or CT scan.	Oral vitamin C 1 g daily and oral vitamin E (400 IU daily + standard treatment regimen.	Hydroxychloroquine 400 mg on the first day followed by 200 mg every 12 h.	Primary: Clinical response of at end of treatment in three ways: cure (complete elimination of clinical symptoms), improvement (elimination of some primary clinical symptoms), and failure (continued or exacerbated primary symptoms). Secondary: Duration of hospitalization, mortality, and change of lab variables.	Hydroxychloroquine or standard regimen as per national policy
5	Darban M et al., 2021 [17]	Adults with severe COVID-19	IV vitamin C (2 g, every 6 hourly), oral melatonin (6 mg, 6 hourly), and oral zinc sulphate (50 mg, 6 hourly) for 10 days + standard therapy	Standard therapy alone	Changes in P/F ratio and inflammatory markers (LDH, ESR, CRP, ferritin)	Azithromycin (250 mg daily); lopinavir/ritonavir (100 mg/25 mg daily); glucocorticoids; Oxygen therapy.
6	Thomas S et al., 2021 [16]	Adult patients COVID-19; multiple treatment factorial trial	Three intervention groups: Group 1-zinc gluconate (50 mg), group 2-vitamin C (8 gm), Group 3: both agents along with standard care	Group 4: Standard care alone	Primary: Days needed to attain 50% reduction in symptoms based on questionnaire (Severity of each following symptoms: I)fever; II) cough; III) dyspnea; IV) fatigue rated on a 4-point scale). Secondary: Days needed to reach a total symptom severity score of 0, cumulative severity score at day 5, hospitalizations, mortality, adjunctive therapies, and adverse effects of the study supplements.	-