The COVID-19 pandemic, caused by SARS-CoV-2, has resulted in millions of cases and deaths globally. Severe COVID-19 infections trigger a significant inflammatory response, often leading to a cytokine storm. Given the immune system's dependence on micronutrients for optimal function, including vitamin C, this review investigates the potential of vitamin C as a prevention and treatment modality. Current COVID-19 treatments include repurposed drugs, but these can be costly. Vitamin C therapy, if proven effective, offers a potentially cost-effective alternative or supplement. This study critically assesses the evidence supporting vitamin C's role in immune function and its potential application in combating COVID-19.

The literature review involved searching PubMed using keywords such as "COVID-19," "coronavirus," "Vitamin C," "nutrition," "pneumonia," and "sepsis." The study focused on randomized controlled trials and clinical trials published in 2020 and 2021, supplemented by background information from articles published within the last 20 years. Articles were screened for relevance to the study's topic.

The research methodology involved a comprehensive literature review of existing studies on vitamin C and its relation to COVID-19. The review primarily focused on randomized controlled trials and clinical trials published in 2020 and 2021 to ensure the most up-to-date information. PubMed was used as the primary database, and searches were conducted using various relevant keywords related to COVID-19, vitamin C, immune function, and associated complications. The selected articles underwent a screening process for relevance to the study's objectives. Background information was also gathered from relevant articles published in the past twenty years, providing a broader contextual understanding of vitamin C's role in immune function and overall health. The authors aimed to synthesize the findings from these studies to present a comprehensive overview of the current evidence regarding the effectiveness of vitamin C in the context of COVID-19 prevention and treatment.

Vitamin C's importance in immune function is well-established. It significantly impacts various immune cells, including neutrophils and lymphocytes, enhancing their chemotactic activity, phagocytosis, and immunoglobulin production. It also acts as a potent antioxidant, protecting immune cells from reactive oxygen species (ROS) and reducing oxidative stress. Vitamin C is essential for collagen synthesis, vital for tissue repair and wound healing. Prophylactic vitamin C use has shown promise in reducing the severity and duration of non-COVID-19 respiratory infections, particularly in individuals with vitamin C deficiencies. However, evidence for vitamin C's prophylactic role against COVID-19 itself is limited, with some studies showing no significant benefit in individuals without a deficiency. Supplementing vitamin C in those with deficiencies, particularly the elderly, might help, but further studies are needed. The efficacy of vitamin C as a treatment for COVID-19 is also unclear. While some studies showed positive outcomes, such as decreased inflammatory markers and improved oxygen saturation, others found no significant improvement in mortality or other key outcome measures like ICU stay. In the treatment of COVID-19 complications, some studies suggest benefits in treating sepsis but not ARDS or pneumonia. There is evidence that high-dose IV vitamin C might reduce mortality in sepsis, but findings on its effectiveness are varied across studies.

The extensive evidence supporting vitamin C's role in immune function and its antioxidant properties provides a theoretical basis for its potential use in COVID-19. However, the results of clinical trials evaluating vitamin C's efficacy in preventing or treating COVID-19 are inconsistent and often inconclusive. While some studies show potential benefits in specific aspects, such as reducing inflammation or improving oxygen saturation, a definitive conclusion regarding its overall efficacy as a treatment remains elusive. The mixed results may be due to factors like variations in dosage, administration methods, patient populations, and study designs. More rigorously designed and larger-scale clinical trials are needed to clarify vitamin C's role in COVID-19 management.

Currently, there is insufficient evidence to definitively recommend vitamin C for the prevention or treatment of COVID-19. While its vital role in immune function and its antioxidant properties are well-established, the clinical data supporting its use against COVID-19 are mixed and require further investigation. Maintaining adequate vitamin C intake through a balanced diet or supplementation to ensure optimal immune function remains advisable, but high-dose vitamin C therapy should not replace existing effective treatments for moderate-to-severe COVID-19. Future research should focus on larger, well-designed clinical trials to assess the efficacy and safety of vitamin C in diverse patient populations and various treatment contexts.

The review's limitations include the reliance on published literature, which might not represent the full spectrum of research. The heterogeneity of study designs, dosages, and patient populations across the included studies makes it challenging to draw definitive conclusions. Furthermore, the review focuses primarily on trials conducted in 2020 and 2021, and more recent research might offer a different perspective.