(non)impact of placebo on the common cold

Over on SciBlogs(NZ), Elf has an interesting post about rhinoviruses, the causal agent for the common cold. I’ve just read it  & thought it particularly apt in light of a recent paper on the impact of placebo treatments on the duration of cold symptoms (hat tip to the inimitable Mark Crislip).

The Medscape review for the research (you need to sign up to read this, but registration is free; you can also find the paper on PubMed) is entitled Placebo effects modest in treating the common cold, which is probably slightly generous given the actual research findings.

The project enlisted 719 individuals who had just come down with symptoms of the common cold. They were placed in one of 4 treatment groups. Those in the first group weren’t given any pills (although one hopes they did receive a certain amount of sympathy!). Group 2 were blinded as to whether or not they were receiving a placebo. Group 3 were blinded for echinacea (another placebo?), and the members of the 4th group got ‘open-label’ echinacea.  In addtion, when they were first enrolled in the study, 120 participants were asked to give their views on the effectiveness of echinacea as a treatment for colds, rating it on a scale of 0-100 where 100 = extremely effective.

While patient reports on the duration & severity of their illness were necessarily subjective, the researchers also measured white blood cell (neutrophil) counts and levels of interleukin 8 in fluids obtained by washing the patients’ nasal cavities (which doesn’t bear thinking about, although I suppose it might momentarily relieve that snotty feeling…)

So what did the research team find?  There was no statistically significant difference in average overall severity scores – between 3 and 8 hours difference, depending on the patient groups. Or, to quote from the Medscape review:

Participants randomly assigned pills reported colds that were on average 0.16 to 0.69 days days shorter and 8% to 17% less severe than the colds of patients in the no-pill group.


There were no statistically significant between-group differences in mean global severity scores.

Just as tellingly, using the objective end-points (measures of immune system activity) there was no difference between the four ‘treatments’. And just as interesting: those with a belief that echinacea works as a treatment for colds reported shorter & less severe illness regardless of whether their pills actually contained echinacea, or whether patients just thought they did. But again, perceived differences weren’t statistically significant. Which I guess shows the power of placebo, but also says nothing about the value of echinacea. (See also this Cochrane review.)

So for the authors to say that “Participants randomized to the no-pill group tended to have longer and more severe illnesses than those who received pills" is putting an extremely positive spin on a non-result.

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