Common Cold

Vitamin C

In 2013 The Cochrane Database of Systematic Reviews published a meta-analysis of 29 trials involving 11,306 participants which found that, while regular vitamin C supplementation could reduce the duration and severity of colds, it was not effective in preventing of the common cold. [Source]
In 2006 Annals of Nutrition & Metabolism published a review of various clinical trials for which the authors concluded that adequate intake of vitamin C and zinc can reduce the severity and duration of cold symptoms while also reducing the incidence of pneumonia, malaria and diarrhea, particularly in children in developing countries. [Source]

On 2001 Advances in Therapy published a study including 146 volunteers who received either an allicin-containing garlic supplement or a placebo for a 12-week period between November and February (cold season). The garlic group contracted significantly fewer colds and recovered faster if infected, leading researchers to conclude that allicin-contain garlic supplements can help prevent common cold virus infections. [Source]
In 2012 Clinical Nutrition published a 120-subject, double-blind, placebo-controlled study which tested 2.56 grams of aged garlic extract per day on the body’s immune response to the common cold virus. After 45 days the garlic group had greater concentrations of natural killer cells (white blood cells vital to immune function) and experienced reduced severity in cold symptoms. [Source]

In 2005 the Canadian Medical Association Journal published a double-blind study in which 279 subjects with a history of at least two colds in the previous year took either 2 capsules of North American ginseng or placebo daily for a period of 4 months. The results showed that subjects in the ginseng group had a significant reduction in number of colds and severity of cold symptoms during the study period. [Source]

In 2016 Nutrients published a double-blind study involving 312 economy class passengers which showed that black elderberry reduced cold duration by 51 percent and cold severity by 58 percent in air travelers when compared to a placebo. [Source]