From FT.com [1], here are the number of new confirmed Covid-19 cases, per million residents, averaged over the last 7 days:
- Maine: 130
- Washington State: 230
- Idaho: 740
- Montana: 1000
- South Dakota: 1600
From this data, I conclude that mask use is effective at reducing the spread of covid-19.
[1] https://ig.ft.com/coronavirus-chart/?areas=eur&areas=usa&are...
As the weather turned colder people started staying indoors and the conditions for transmitting the disease turned more favorable. (Eg restaurants, etc.)
Plus after a critical mass regular (intuitive) precautions are not enough. Eg in France restaurants allowed people to dine if they looked okay, and had a mask on while waiting for a table, and while not actually eating.
Clearly this was too lax. (aug 31 https://www.connexionfrance.com/French-news/Masks-in-Paris-r... )
[1]https://www.researchgate.net/publication/339321846_Impact_of...