Statistically significant lower risk is seen for mortality, ICU admission, hospitalization, and recovery. 23 studies from 23 independent teams in 12 countries show statistically significant improvements.
Meta analysis using the most serious outcome reported shows 21% [14‑27%] lower risk. Results are similar for Randomized Controlled Trials, higher quality studies, and peer-reviewed studies. Clinical outcomes suggest benefit while viral and case outcomes do not, consistent with an intervention that aids the immune system or recovery but may have limited antiviral effects. Early treatment is more effective than late treatment.
Results are robust — in exclusion sensitivity analysis 28 of 70 studies must be excluded to avoid finding statistically significant efficacy in pooled analysis.
6 RCTs with 1,420 patients have not reported results (up to 3 years late).
The European Food Safety Authority has found evidence for a causal relationship between the intake of vitamin C and optimal immune system function Galmés, Galmés (B).
No treatment or intervention is 100% effective. All practical, effective, and safe means should be used based on risk/benefit analysis. Multiple treatments are typically used in combination, and other treatments are more effective. The quality of non-prescription supplements can vary widely and the quantity of the active ingredient may be significantly lower than stated Chanyandura, Crawford, Crighton.
All data to reproduce this paper and sources are in the appendix. Other meta analyses show significant improvements with vitamin C for mortality Bhowmik, Kow, Kow (B), Olczak-Pruc, progression Sun, severity Bhowmik, and cases Xu.
Covid Analysis et al., May 2024, preprint, 1 author.