A Cochrane review [Abstract] 1 included 16 studies with a total of 22 comparisons of Echinacea preparations and a control group (19 placebo, 2 no treatment, 1 another herbal preparation). The majority of trials had reasonable to good methodological quality. Three comparisons investigated prevention (411 participants) and 19 comparisons treatment of colds (1 910 participants of which only 1 064 actually started treatment; 846 did not catch a cold during the study period). Two studies were in children, the remaining 14 in adults. A variety of different Echinacea preparations were used.
None of the prevention trials showed an effect over placebo. Comparing an Echinacea preparation with placebo as treatment, a significant effect was reported in nine comparisons, a trend in one, and no difference in six. The number of participants who developed the full picture of a cold was significantly lower in the one out of two trials with Echinacea (RR 0.67; 95% CI 0.46 to 0.97, n=120), while in the other trial the difference just missed statistical significance (RR 0.88; 95% CI 0.76 to 1.00, n=80). The severity and duration of cold symptoms was reduced over placebo in a study using pressed juice from the aerial parts of E. purpurea (SMD -0.55, 95% Cl -1.00 to -0.11, n=80), while the other five studies including other preparations failed to show significant differences. From the trials that included some sort of sum of symptoms score as an outcome measure significant differences were detected after treatment days two to four in 2/6 trials: high dose of E. purpurea root extract (SMD -1.28, 95% Cl -1.67 to -0.88, n=120) and in one of the two trials of the E. purpurea preparation standardized on three bioactive components (SMD -0.52, 95% Cl -0.88 to -0.17, n= 120), and after treatment days five to ten in 5/10 trials. For nasal symptoms like runny nose significant differences over placebo were found for the E. purpurea preparations after two to four days of treatment in 2/6 trials, and after five to fo ten days of treatment in 3/10 trials. A trial of E. pallida root extract reported a highly significant result regarding duration of colds (SMD -1.83 95% Cl -2.20 to -1.46) whereas a trial of an Echinacea mixture found no difference compared to placebo. Evidence from more than one trial was available only for preparations based on the aerial parts of E. purpurea.
Comment: The quality of evidence is downgraded by inconsistency (variability in results across studies) and by indirectness (differences in studied interventions).
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