Zinc and the common cold

Today's big health news story is a new Cochrane review that looked at zinc as a treatment for the common cold. The conclusion of the review is that taking zinc supplements within 24 hours of the onset of a cold can reduce the duration and severity of symptoms.

However, I'm not convinced.

The finding is based on a relatively small number of studies. Only 6 studies were used in the meta-analysis of the duration of symptoms. 4 of those studies were rather unimpressive, showing either non-significant reductions in the duration of symptoms, or a reduction of less than 1 day. The overall finding of a significant reduction in symptom duration was largely driven by 2 small studies (48 and 50 patients respectively), which found much larger reductions.

This is consistent with a pattern known as publication bias: small studies with significant positive results get published, small studies with negative results don't get published. It's possible that other small studies have been done, but not published, because their results were negative, and that inclusion of those studies would move the overall results to being negative.

Assessment of publication bias is an important part of a systematic review. Oddly, the methods section of the Cochrane review tells us that they looked for publication bias, but I can't find anywhere in the results section a description of what they found.

I think that's quite important here. Without knowing more about how likely publication bias is (and a quick glance at the results suggests it is quite likely), it's hard to know whether to believe these results. I am not convinced that zinc supplementation is truly a useful treatment for the common cold. Further research, as they say, is needed.