The efficacy of mask mandates - Printable Version
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The efficacy of mask mandates - BostonCard - 10-05-2020
Clever study. Because Ontario staggered its implementation of mask mandates over the course of 2 months, it was possible to see if the introduction of the mandate made a difference.
https://www.nber.org/papers/w27891
Quote:We estimate the impact of mask mandates and other non-pharmaceutical interventions (NPI) on COVID-19 case growth in Canada, including regulations on businesses and gatherings, school closures, travel and self-isolation, and long-term care homes. We partially account for behavioral responses using Google mobility data. Our identification approach exploits variation in the timing of indoor face mask mandates staggered over two months in the 34 public health regions in Ontario, Canada's most populous province. We find that, in the first few weeks after implementation, mask mandates are associated with a reduction of 25 percent in the weekly number of new COVID-19 cases. Additional analysis with province-level data provides corroborating evidence. Counterfactual policy simulations suggest that mandating indoor masks nationwide in early July could have reduced the weekly number of new cases in Canada by 25 to 40 percent in mid-August, which translates into 700 to 1,100 fewer cases per week.
It's still an observational study, so it could be biased, but it gets around the problem of measuring mask usage.
BC
RE: The efficacy of mask mandates - M T - 11-10-2020
Thanks for finding this. I didn't get a chance look at it until Goose brought it up again (Thanks!).
The authors find
Quote: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.
RE: The efficacy of mask mandates - OutsiderFan - 11-10-2020
Do we have a universal definition of what "wearing a mask" means exactly?
IOW, does mask wearing in the context of this and other studies apply to having a mandate in place vs. not having one in place, and then, what defines mask wearing? Is it anytime I am not in my house? Not when I am outside by myself but anytime I am around others, inside or outside?
I had some family members come in my mom's house this weekend and wore masks, but my mom and I didn't wear one. I go out walking by myself and don't wear one, but when I go to the store or in a place with people I don't know - in side or out - I do wear one. I assume I qualify as wearing a mask, but am not altogether certain.
RE: The efficacy of mask mandates - fullmetal - 11-10-2020
At the very least, I would say masks indoors at all times and masks outdoors when remaining within personal comfort distance, which for me, is something on the order of 10 ft.
I've gone on trail runs recently running by folks without masks, and as long as we're moving quickly past each other, I'm okay with it. I might hold my breath for a beat as I pass by them. Personally, I don my mask when I see other runners approaching, and I'll take off my mask after passing them. The mask itself doesn't hinder my breathing, but it does fog up my glasses.
RE: The efficacy of mask mandates - M T - 11-10-2020
The study spoke of the mandates for indoor mask wearing.
As I indicated earlier, at walking pace & quiet breathing, you exhale the volume of a 4" diameter sphere of air every 18' (exhalation spread over about 9'). In cooler weather, I believe this air quickly rises above your head before it moves away from your head. If exercising, you'll blow a larger volume of air forward and down but I didn't estimate how much or how often.
I've mostly focused on the aerosol particles, but it has been demonstrated the droplets can go much further than even 10' with the right breeze (whether from a fan or nature). While the volume in which they exist rises as the cube of the distance, I believe they tend to remain a cloud of droplets. (I recently put a blue-colored liquid chemical in a pool with a decent flow of water from the pump. I could still see the blue (very diluted, but visible) as it circulated around the diameter of the pool and came back to me after traveling 30 yards, in a minute or two.)
While you may not need your mask if you are outdoors, remember that its purpose is to block the virus particles, not disinfect them. Every time you take it off or put it on, you will flex the material and wave it in the air or drop it on a surface, possibly raising a cloud of what is on the inside or outside surface, plus the possibility of touching the surface. I've not seen anyone study the surface of masks, except one study that showed virus was found on the outside of masks of infected persons who had coughed in their mask. I'd worry about that if I were eating at a restaurant and repeatedly putting on & off my mask.
I normally wear a KN95 mask, but recently used a surgical mask for a day. I found it to feel like no mask at all. Air could find its way around the mask as I breathed in and out. I have done manual labor in a KN95 mask in 100F weather and found that I had to take mask breaks (in my car) to catch my breath and let the moisture (which impedes air flow) evaporate from the inside of the mask.