Thread by @jburnmurdoch

The good thing about this story is that any time I see someone sharing it as evidence that wearing masks cuts Covid incidence by 53%, I know they haven’t read the paper it’s based on and aren’t fussed about the quality of evidence as long as it produces the right number 🙃

What do I mean by this?

Let’s start by saying that yes, we have good evidence that masks reduce Covid incidence 😷🦠📉😀

BUT it points to nowhere near a 53% reduction. As today’s @bmj_latest states, the best evidence — randomised controlled trials — point to more like a 10% cut

Indeed, here is the BMJ’s take on today’s paper:
• Most of the studies included in the meta-analysis from the Guardian story are poor quality and subject to major biases
• We need more & better research [like the mask-wearing RCTs which have consistently found a smaller effect]

To conclude
• Based on best evidence, masks cut incidence by ~10% ✅
• So masks are a good, low-cost measure to use on top of vaccines ✅
• If you care about finding the most effective interventions, don’t share obviously misleading numbers based on "very low quality" evidence

Here’s the study behind the Guardian "53%" story bmj.com/content/375/bm…

And here’s today’s BMJ editorial setting out the substantial limitations to that study, the dearth of good studies + evidence, and what the best current evidence on masks actually is bmj.com/content/375/bm…

Finally: this stuff grates so much because it’s often people on "my side" sharing what is essentially misinformation.

We would rightly call this out if people on "the other side" did it, and we diminish trust in statistics, science and journalism when we fall into the same traps