Hello and welcome to more or less on the BBC World Service. We're the programme that

unmasks suspicious statistical citations, introduces fresh air and maintains a social

distance from misinformation and I'm Tim Hafen.

Last week this claim was doing the rounds in news outlets including the Guardian and Forbes.

The mask wearing linked to 53% cut in Covid incidents, global study finds.

The articles cited a paper published in the British Medical Journal.

Now here at more or less we know we can't have nice things, not these days.

While there are plenty of reasons to be confident that masks help prevent the spread of airborne

viruses, the idea that a little piece of cloth would more than half your risk of catching

a potentially fatal disease, well it seemed a little too good to be true. We had questions

and we put them to Professor Linda Sharples, Head of Medical Statistics at the London

School of Hygiene and Tropical Medicine and Professor Andrew Hayward, Director of the Institute

of Epidemiology and Healthcare at University College London to find out more.

Here's Andrew Hayward.

Well the study was actually a review of all sorts of different interventions to try and reduce

Covid transmission and one of which was mask use and what it tried to do was to summarise

research from across a number of different studies to look at that and it did that using

an approach called Metroanalysis which is usually used when we've got a lot of small studies

that aren't quite big enough to give you the answer and you combine them together to

try and give you a more accurate view of what the true answer is.

That seems fairly sensible to me so I asked Linda Sharples, did the study find that wearing

a mask causes a 53% reduction in Covid cases?

...