The study Preventing the common cold with a garlic supplement: a double-blind, placebo-controlled survey shows a reduction of days challenged virally by about 70% just by taking 180mg allicin every day.
This would be really huge. The economic impact of the common cold per year seems to be in the billions (Productivity losses related to the common cold.).
But the author of the allicin study (Peter Josling) appears to be involved in selling garlic supplements, so I'm not sure if one can trust his study fully.
The only related study I found is: Supplementation with aged garlic extract improves both NK and γδ-T cell function and reduces the severity of cold and flu symptoms: a randomized, double-blind, placebo-controlled nutrition intervention.
Are there any further studies verifying or refuting the existence of this effect?