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What to draw from Macintyre et al 2015?

by ChristianKl1 min read5th Apr 20206 comments



Last week it seems the West decided that we should all wear masks and wear cloth masks if we don't have proper masks.

As Scott Alexander pointed out, Macintyre et al 2015 seems to be a only controlled trial of cloth masks and it writes:

For example, a contaminated cloth mask may transfer pathogen from the mask to the bare hands of the wearer. We also showed that filtration was extremely poor (almost 0%) for the cloth masks. Observations during SARS suggested double-masking and other practices increased the risk of infection because of moisture, liquid diffusion and pathogen retention. These effects may be associated with cloth masks.

I'm not on record for being the biggest fan of evidence-based medicine, but getting everybody to take use a medical intervention based on pathotheoretical reasoning when the paper that provides out best evidence cautions against that intervention is risky.

Why does Macintyre et al 2015 come to so different conclusion about the ability of cloth masks to filter then other studies?

What action could people take to reduce the potential increase risk that Macintyre sees when they don't have better masks?

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The authors published a clarification, in it they recommand:

There are now numerous reports of health workers wearing home made cloth masks, or re-using disposable mask and respirators, and asking for guidance. If health workers choose to work in these circumstances, guidance should be given around the use.
There have been a number of laboratory studies looking at the effectiveness of different types of cloth materials, single versus multiple layers and about the role that filters can play. However, none have been tested in a clinical trial for efficacy. If health workers choose to work using cloth masks, we suggest that they have at least two and cycle them, so that each one can be washed and dried after daily use. Sanitizer spray or UV disinfection boxes can be used to clean them during breaks in a single day. These are pragmatic, rather than evidence-based suggestions, given the situation.

Unfortunately that study doesn't cover the question we want answered:

The control arm was ‘standard practice’, which comprised mask use in a high proportion of par- ticipants. As such (without a no-mask control), the finding of a much higher rate of infection in the cloth mask arm could be interpreted as harm caused by cloth masks, efficacy of medical masks, or most likely a combination of both

So the study found evidence that surgical masks are better than cloth masks, but doesn't say anything about whether cloth mask are better than nothing.

The study authors speculate that cloth masks are detrimental, but after reading through the study again, I'm pretty sure they provide absolutely no evidence for that (2 out of 1607 people in their study didn't wear masks).