So, The Big Mask study has been published, and I thought rather than expound on what the results DID show (everyone's doing that), I might point out a few things that they DIDN'T show 1/n
2/n Study is here, as ever have a read. A very simple, nicely done RCT comparing the advice to wear masks with no such advice in Denmark: https://www.acpjournals.org/doi/10.7326/M20-6817
3/n So, first point. This study says NOTHING about whether mask mandates are good public health policy
Indeed, the authors themselves point this out in the discussion
Indeed, the authors themselves point this out in the discussion
4/n In fact, we already knew that telling people to wear masks (and providing them with masks) is a relatively ineffective way to stop them from getting a respiratory disease. Universal masking is more complex than that
5/n The study DOESN'T show that masks are ineffective at preventing infection for the person wearing them
I know it sounds weird, but it's true!
I know it sounds weird, but it's true!
6/n See, the authors powered their study (i.e. recruited participants) assuming that masks decreased your risk of infection by 50%, which is quite a lot!
7/n What the study actually shows is that advising someone to wear a mask doesn't reduce their risk of COVID-19 by ~50% or more~, but the results don't exclude smaller benefits
8/n A reduction in risk of infection of 20% would be a bit meaningless to the individual but HUGE at a population level, so this is not a minor point
9/n The study also DOESN'T show that WEARING A MASK is ineffective. It showed that PROVIDING MASKS AND TELLING PEOPLE TO WEAR THEM was ineffective ON TOP OF SOCIAL DISTANCING
10/n As the authors note, compliance was pretty poor. Lots of people were told to wear masks, but didn't
Hard to say what this means for an individual wearing a mask 24/7
Hard to say what this means for an individual wearing a mask 24/7
11/n Moreover, there was a lot of social distancing already going on in Denmark at the time - this means that we can't really say that MASKS are ineffective but rather than masks didn't reduce infection numbers significantly on top of social distancing
12/n Again, this is not a minor point - masks may indeed not reduce infection numbers much during lockdown, but that doesn't say a lot about their effectiveness at other times
13/n Ok, a technical addition that is nevertheless important. The authors do not report correcting their result for the test sensitivity and specificity of their serology test
14/n Serology tests are used to find antibodies, and they are (as all tests are) imperfect
So, usually we correct for the imperfections to get a better estimate of the true number of people with antibodies
So, usually we correct for the imperfections to get a better estimate of the true number of people with antibodies
15/n In this case, the study found that 1.8% of people in the mask group had antibodies, compared to 2.1% of people in the non-mask group
But those are just the RAW figures
But those are just the RAW figures
16/n If we use the Rogen-Gladen estimator, which is a pretty standard correction for test characteristics, we see instead that 1.59% and 1.95% of people in masks/no masks were probably infected, respectively
17/n This sounds like a minor point, but it actually isn't - if only 1.59%/1.95% of people were infected, it means that the study was underpowered for its main analysis, and thus we can't conclude much from the results
18/n Sorry, small correction - I used the final totals of 1.8% and 2.1% not the actual antibody numbers of 1.6% and 1.7% in that calculation. If you apply the correction properly, you get 1.56% masks and 2.09% non-masks
19/n For some context, to find a difference this small, the study would've needed to recruit about 24,000 people, or 12,000 in each group, which is about 4x as big