The bottom line: taking Echinacea while you are still healthy may help prevent the common cold, but Echinacea does not seem to be of significant benefit for treating colds that have already developed.
Evidence for treatment of common colds: Although some smaller, lower quality studies suggest Echinacea purpurea may help reduce severity and duration of common colds, higher quality clinical research indicates that Echinacea does not reduce severity or duration of colds compared to placebo. Additionally, Echinacea purpurea does not seem to be effective in children.
Evidence for prevention: Several meta-analyses have shown significant reductions (ranging from 10-58%) in the occurrence of common cold when Echinacea is taken consistently.
Caution: It is also important to note that products containing Echinacea tend to vary significantly in purity, dose, and effectiveness.
References:
1. Jawad M, Schoop R, Suter A, Klein P, Eccles R. Safety and Efficacy Profile of Echinacea purpurea to Prevent Common Cold Episodes: A Randomized, Double-Blind, Placebo-Controlled Trial. Evidence-Based Complementary and Alternative Medicine [Internet]. 2012;2012:1-7. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3457740/
2. Karsch-Völk M, Barrett B, Kiefer D, Bauer R, Ardjomand-Woelkart K, Linde K. Echinacea for preventing and treating the common cold. Cochrane Database of Systematic Reviews [Internet]. 2014;. Available from: https://www.cochranelibrary.com/cdsr/
3. Schoop R, Klein P, Suter A, Johnston S. Echinacea in the prevention of induced rhinovirus colds: A meta-analysis. Clinical Therapeutics [Internet]. 2006;28(2):174-183. Available from: https://www.clinicaltherapeutics.com/article/S0149-2918(06)00041-5/pdf