"The factor by which β was reduced was conservatively set to 2."
"Varying degrees of mask effectiveness are modelled by the mask transmission rate T and mask absorption rate A, which denote the proportion of viruses that are stopped by the mask during exhaling (transmission) versus inhaling (absorption), respectively. Weset T = 0.7 and A = 0.7 to model the use of inexpensive, widely available, and even nonmedical or homemade masks [...]"
So basically, they assume certain values for how effective masks are, and then simulates the dynamics of the epidemic. Nowhere do they try to justify the specific values chosen.
I expect that the conclusions cited (if only 40% wear masks, there is little benefit; if 80% wear masks, total cases go down by a factor 1/12) to be heavily dependent on those assumptions, and so I would be cautious to read too much into it.
Link to paper: https://arxiv.org/pdf/2004.13553.pdf
If only 40% were to wear masks, they found almost no benefit.
By "masks" they don't just mean N95 masks. They include surgical masks, scarfs and bandanas used as masks, and other homemade or improvised masks.
Here's an article about that study [1].
For those who don't want to make their own or improvise, masks are becoming reasonably available. Newegg, for example, has several sellers which a search there on "masks" will find [2]. This includes some with stock in the US ready to ship, with delivery in a week or less. I just bought 50 disposable surgical-style masks for $30 from this listing [3], and they arrived in just over a week. At the time I bought they shipped from the seller but since then Newegg is handling shipping.
I picked that seller because they shipped from the US. A lot of sellers ship from China. Many of those do have reasonable shipping time, but I have no idea if the US Customs and Border Patrol is still seizing and redirecting PPE supplies that cross their paths. I figured a shipment that was entirely domestic was less likely to fall prey to government fuckwittery.
Tom's Guide has been maintaining a list of places with masks avaiable, which they update frequently [4]. That was were I learned that Newegg had masks in stock.
[1] https://www.vanityfair.com/news/2020/05/masks-covid-19-infec...
[2] https://www.newegg.com/p/pl?d=masks
[3] https://www.newegg.com/picotee-face-mask/p/0CE-02AC-00001
[4] https://www.tomsguide.com/news/where-to-buy-face-masks-and-c...