It is a well-known fact that a balanced diet which has adequate quantities of vitamin C leaves a positive effect on the immune system and reduce the chance of infections. It has been suggested that in people who are at high risk of infection, vitamin C supplementation can modulate inflammation, with potential positive effects on immune response to infections. 

Study has shown that a combination of a small, long-term daily dose of vitamin C (no more than 1g/day) to sustain immunity and a larger dose of vitamin C during the onset of the common cold (about 3-4 g/day) was related to the ability to relieve chest pain, fever, and chills, minimizing the duration to stay indoors, and the mean duration of the disease. Vitamin C has been reportedly beneficial in people under heavy physical stress, and pneumonia. Based on critical illness studies, vitamin C infusion has been hypothesized as a treatment for COVID 19 patients. Optimal nutritional status including vitamin C can effectively reduces inflammation and oxidative stress, improve the immune system regulation. 

Resource: Cerullo G, Negro M, Parimbelli M, Pecoraro M, et al. The ling history of vitamin C: from prevention of the common cold to potential aid in the treatment of COVID-19. Frontiers of immunology. 2020; 11: Article 574029.