Full Version: "Mean" Mask wearing decreases R by (only?) 19%
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(I had to quote "Mean" in the subject.  Otherwise to easy to read it as wearing mean masks!)

Mask wearing in community settings reduces SARS-CoV-2 transmission

Quote:We resolve conflicting results regarding mask wearing against COVID-19. Most previous work focused on mask mandates; we study the effect of mask wearing directly. We find that population mask wearing notably reduced SARS-CoV-2 transmission (mean [across regions of the world] mask-wearing levels corresponding to a 19% decrease in R). We use the largest wearing survey (n = 20 million) and obtain our estimates from regions across six continents. We account for nonpharmaceutical interventions and time spent in public, and quantify our uncertainty. Factors additional to mask mandates influenced the worldwide early uptake of mask wearing. Our analysis goes further than past work in the quality of wearing data–100 times the size with random sampling–geographical scope, a semimechanistic infection model, and the validation of our results.

They are measuring R across the population 24x7, but mask wearing is only being measured when worn in public.  Wearing masks only reduces transmission between households, not within households.  The use of masks in public is no barrier to transmission (ie, doesn't reduce R) at home.   So, I'd argue that the reduction in R  while in public  is much more pronounced.

I suspect one could model the population as a group of household units as being composed of N people, with a particular R inside the household and a separate R outside the household ("in public"), where each of the N is independently at risk of bringing COVID into the house.  The combined value of those R's is being measured as a reduction of 19%, but the reduction in R outside the household would be more significant.

They do point out that some of their data can't distinguish between people wearing cotton masks 51% of the time vs wearing a N95 100% of the time.  They also seem to recognize that actual mask wearing was less than self-reported.

It took a while for me to realize that their %R reduction doesn't account for actual mask wearing.   I wasn't able to find which region had what reduction.  The two regions that had about 1% reduction might be regions that had almost no mask wearing at all.

They noted that mandates did not reflect step functions in usage (at start or end).   I will use the analogy of training cats.  You can tell a cat to do something if it already is doing it.  Maybe it is a bit of a positive enforcement, but it seems that the mandate didn't have a sharp effect on practices.  

They have graphs of mask wearing from 3 weeks before to 3 weeks after a mandate for various countries (Netherlands, England, Scotland, N. Ireland  are by far the least masked at the time of mandate) and US States (virtually every US state listed wore masks more than all countries except a handful (S. Korea; Brazil; Italy, Turkey, and a few more)).   My interpretation of that is that the mandates came too late in the US, after the population was already mostly convinced it was necessary and was doing it.  The data show that there was almost no noticeable effect in mask wearing by the mandates.  Either that, or it didn't effect the level of lying about mask wearing.    HOWEVER, note that the graphs in the supplement of mask wearing show a steep increase at or close to mandate date.

Their "mean" is among 92 regions of the world, regardless of population (Delaware and India are each one region). 46 regions had a reduction in R of 18-23%; 46 regions had a reduction of 1-18%.  Note that their reduction estimates were for mask wearing alone, after accounting for other factors.