Hart A, Dey P. Echinacea for prevention of the common cold: an illustrative overview of how information from different systematic reviews is summarized on the Internet. Prev Med. April 21, 2009: [Epub ahead of print].

Many small, single-center, randomized controlled trials have examined complementary medicines, and a number of systematic reviews have summarized the overall evidence of complementary therapies. The conclusions of systematic reviews depend on the decisions made regarding which trials to include and whether or not the results of the trials are statistically combined in a meta-analysis. People who use complementary medicines may not have access to the scientific literature, but many of them can access the World Wide Web. These authors examine three systematic reviews of echinacea (Echinacea spp.) for the prevention of the common cold to illustrate how the selection criteria for the trials in the reviews could lead to different conclusions and to illustrate the ways in which Web page authors use the reviews to construct advice about echinacea.

The three systematic reviews examined by the authors include a review of three trials by Schoop et al in 2006¹ (See HC 030561-301), a Cochrane review that includes a summary of three comparisons from two trials (Linde et al in 2006²; See HC 020161-299), and a review by Shah et al in 2007³ (See HC 080571-336) of nine trials including the subsets of trials included in the other two reviews. Schoop et al address prevention of induced rhinovirus colds, while Linde et al and Shah et al examine both prevention and treatment of the common cold.

Shah et al concluded that the published evidence "supports echinacea's benefit in decreasing the incidence and duration of the common cold." According to Schoop et al, the meta-analysis "suggests that standardized extracts of echinacea were effective in the prevention of symptoms of the common cold after clinical inoculation, compared with placebo." And, Linde et al concluded that "echinacea preparations tested in clinical trials differ greatly" and that "beneficial effects of other echinacea preparations, and echinacea used for preventative purposes might exist but have not been shown in independently replicated, rigorous RCTs." All three systematic reviews concluded that more high-quality trials are needed on the prophylactic efficacy of echinacea.

The authors used a funnel plot to illustrate the heterogeneity related to the trials' selection criteria. The observed clinical heterogeneity may explain why authors of the three reviews vary as to whether they consider that the preventive efficacy of echinacea has been established. Schoop et al restricted their study to trials investigating the efficacy of standardized extracts of echinacea used in "the prevention of symptoms of the common cold after clinical inoculation" of single strains of rhinovirus. "The findings of the review may have less relevance to the prevention of colds acquired in the community," say the authors. The Cochrane reviewers took a strict approach to answer a precise question about echinacea itself in naturally acquired colds. This was likely to give an uncertain result as "the more rigorous the review, the less evidence there will be that the intervention is effective."⁴ Shah et al "took an inclusive approach to the selection of trials that appears to reduce uncertainty in the overall estimate of effect from the meta-analysis but increases uncertainty about what that estimate means," write the authors.

The authors used five major search engines to locate Web pages that referred to the efficacy of echinacea in preventing the common cold and that also referred to each of the three systematic reviews. For each Web page, the authors sought to determine the overall advice about the efficacy of echinacea, how the information from each of the reviews is used, and how the differences among the types of trials included in the reviews are discussed.

They identified 12 Web pages on 11 Web sites, including those of national health and regulatory agencies, publishing houses, 'nutraceutical' and fitness companies, not-for-profit and noncommercial organizations, and an independent trust.

Of the 12 Web pages, four provide consumer information, two provide information for professionals, and one provides information for retailers. Two contained journal papers, one was a consultation on product licensing, one was a press statement, and the remaining source was a comment in a chat room. Three Web pages presented or summarized the findings of all three reviews (including a 2007 HerbalGram press release: http://cms.herbalgram.org/press/2007/usatodayechinacea.html); five pages either presented or summarized the findings from one or two of the reviews and either made a general statement about or just mentioned the existence of the remaining reviews; and four pages used all three reviews as general supporting references. The different conclusions of the reviews and the clinical heterogeneity of the trials in the reviews were not always discussed on the Web pages.

The authors report that the overall advice on the Web pages varied: views expressed included that echinacea did prevent common colds, that it had only modest impact, and that it did not prevent colds. These different conclusions can be difficult for consumers to interpret, especially because of the gaps between the body of evidence and the diverse summaries on the Internet, according to the authors.

Research suggests that consumers may ignore information from Web sites that they perceive as less credible, but definitions of credibility vary.⁵ Consumers may also hold prior beliefs about the health properties of echinacea, which may be based on personal or anecdotal experience and/or material from other media.

More high-quality trials are needed in complementary medicine but areas of uncertainty may prevail, say the authors, and it is important to foster a culture in which accepting and discussing uncertainty is possible. Otherwise, consumers may not be able to make informed treatment decisions and will rely on their own beliefs. "Precision is vital in the reporting of uncertainty in scientific findings," say the authors.

―Shari Henson

References

¹Schoop R, Klein P, Suter A, Johnston SL. Echinacea in the prevention of induced rhinovirus colds: a meta-analysis. Clin Ther. 2006;28:174-183.

²Linde K, Barrett B, Bauer R, Melchart D, Woelkart K. Echinacea for preventing and treating the common cold. Cochrane Database of Systematic Reviews. 2006. Issue 1. Art. No. CD000530. doi:10.1002/14651858.CD000530.pub2.

³Shah SA, Sander S, White CM, Rinaldi M, Coleman CI. Evaluation of echinacea for the prevention and treatment of the common cold: a meta-analysis. Lancet Infect Dis. 2007;7:473-480.

⁴Pettigrew M. Why certain systematic reviews reach uncertain conclusions. BMJ. 2003;326:756-758.