[M T]

Thanks for finding this.  I didn't get a chance look at it until Goose brought it up again (Thanks!).

The authors find

Our  estimates  of  the  mask  mandate  impact  across Ontario’s PHUs are equivalent to a 25–31% reduction in weekly cases.
... [Nationally] The most robust result is the estimated effect of mask mandates:  they are associated with a large reduction in weekly case growth of ... a 36 to 46% reduction in weekly cases across the different specifications.

I wanted to know how this translates to a reduction in R.  Using a 33.33% reduction in new cases per week and a generation interval of 5 days, this works out to a 25% reduction in cases per generation.  Ie, R reduces by 25% with mask mandates in the "highly compliant" population of Canada.

Then can we get any rough idea of mask effectiveness for individuals?  The paper reports that about 30% were wearing masks before the mandate and 61% wore masks after the mandate.  It seems likely that the reduction in R is due both to reduction in transmissions by masked individuals and to reduction of risk of masked individuals catching COVID.   However, if we ignore the reduction in transmissions by masked individuals, and assume that wearing a mask results in a linear reduction of risk of catching COVID relative to wearing no mask, I come up with wearing a mask reduces risk of an individual catching COVID by an upper limit of 65% over no mask. (Using a very big ballpark here). 
0.75 = (39% + 61%*M)/(70% + 30%*M)
M = 0.35

Or, to put it another way, you're 1/3 (or higher) as likely to catch COVID while wearing a mask.

[M T]

Thanks for finding this.  I didn't get a chance look at it until Goose brought it up again (Thanks!).

The authors find

Our  estimates  of  the  mask  mandate  impact  across Ontario’s PHUs are equivalent to a 25–31% reduction in weekly cases.
... [Nationally] The most robust result is the estimated effect of mask mandates:  they are associated with a large reduction in weekly case growth of ... a 36 to 46% reduction in weekly cases across the different specifications.

I wanted to know how this translates to a reduction in R.  Using a 33.33% reduction in new cases per week and a generation interval of 5 days, this works out to a 25% reduction in cases per generation.  Ie, R reduces by 25% with mask mandates in the "highly compliant" population of Canada.

Then can we get any rough idea of mask effectiveness for individuals?  The paper reports that about 30% were wearing masks before the mandate and 61% wore masks after the mandate.  It seems likely that the reduction in R is due both to reduction in transmissions by masked individuals and to reduction of risk of masked individuals catching COVID.   However, if we ignore the reduction in transmissions by masked individuals, and assume that wearing a mask results in a linear reduction of risk of catching COVID relative to wearing no mask, I come up with wearing a mask reduces risk of an individual catching COVID by an upper limit of 65% over no mask. (Using a very big ballpark here). 
0.75 = (39% + 61%*M)/(70% + 30%*M)
M = 0.35

Or, to put it another way, you're 1/3 (or higher) as likely to catch COVID while wearing a mask.