We know that the SARS-CoV-2 virus spreads via the respiratory route.
We know that wearing a mask is considered effective in reducing the spread of other illnesses that spread via the respiratory route, both bacterial and viral. In my lifetime this was seen with TB patients.
It is therefore logical to conclude that masks should have an effect in reducing the spread of Covid-19.
No one is going to do a randomized trial, deliberately exposing one group of people to an unmasked Covid patient and exposing another group of people to the same Covid patient wearing a mask, just to prove this. Anecdotal evidence, such as the Springfield, Missouri hair salon, provide some evidence.

The medical experts and scientists have been putting the information, re: masks out there for the last 6 months, or thereabouts. It would seem that anyone seriously and sincerely interested in learning about masks could and should have long ago learned what makes them effective and for what specific purposes these masks are most useful.
I think the person inquiring has had ample opportunity to learn about them if sincere in wanting to know. Therefore, I must question his or her motives in posing the question.

If I understand you right, you are looking for evidence that mask use is beneficial based on a comparison of outcome in different countries/areas. This evidence is too difficult to get. There are too many ifs, buts and howevers, too many variables. And remember Mark Twain’s (or was it Disraeli’s?) words: “There are three kinds of lies: lies, damned lies and statistics”.

I’m not a medical professional. I do have some background in respiratory protection. I’ve had lots of interaction with scientists and researchers in various fields so I have an understanding of scientific testing and the importance of putting tests and resulting data in proper context. And I have very strong opinions on the value of wearing a mask. Here is my take:

I suggest you put aside the search for the evidence you seek. Better to concentrate on mask performance as it relates to the individual. I recognize that here too it’s not all black and white. Many variables – among them design/production of the mask, mask fit, donning/doffing, decontamination, and one of the most important - characterization of the coronavirus-contaminated environments the mask might encounter. That last one is one of the more difficult and unsettled issues.

A research paper I saw gave the virus particle size as around 125 nanometers or 0.125 microns. That’s pretty damned small and way beyond the filtration capability of cloth and surgical masks. But the good news is that you don’t have to filter out individual virus particles to slow down the virus distribution. An infected person transmits it via exhaled air or, more forcefully, by sneezing or coughing. Thus the filtration challenge to the mask is not individual viruses but droplets or aerosols that contain clusters of the virus. So what you are trying to stop is significantly larger than a single virus. I’ve read that droplets, such as found in a sneeze, are 5 to 10 microns; aerosol particles ranging around 0.5-3 microns. But I wouldn’t be surprised if there is a great variation in the published data. If anyone has any good sources please let me know.

The filtration effectiveness of the mask is measured relative to the size of the filter challenge. An N95 respirator will remove 95% of 0.3+ micron particles. What size particles do the various available surgical and cloth face coverings stop? That depends, of course. It’s definitely not 0.3 like the N95s. But what filtration efficiency do we need in order to declare that masks are an effective countermeasure in the fight against coronavirus?

I think by now most reasonable people are convinced that even simple masks properly worn and used by an infected person (including asymptomatic and pre-symptomatic cases) will significantly cut down the amount of contamination that person releases via exhaled air or sneezes and coughs. But what about protection afforded to the wearer of the mask being another incentive for wearing them?

I firmly believe that a mask, properly worn and used in conjunction with distancing and hygiene, will reduce my risk of getting infected. No I can’t quantify the risk reduction, but based on what I know I believe it to be significant. It pains me, therefore, to see constant references to the masks being “useless” in terms of protecting the wearer. Even the experts have made comments close to that, thankfully, less so more recently. I think their reluctance to endorse the idea of masks protecting the wearer is based on concerns regarding proper mask usage and potential relaxation of other mitigation measures because of a false sense of security. But these are issues that can be handled with proper education of the public, can’t they?

So it’s a very complicated problem and accurate quantifiable assessments of the efficacy of masks (especially cloth masks) in reducing the spread of coronavirus is beyond our reach at this time. But we shouldn’t get hung up on an “all or nothing approach”. We can’t yet eliminate the Covid-19 infection threat. But we can surely reduce the risk: Distancing + Hygiene + Masks + Common Sense

(With regard to Sweden: A July 21 opinion piece by 25 Swedish doctors and scientists characterizes the Swedish approach as a failure. It concludes with “don’t do it the Swedish way.” - https://www.usatoday.com/story/opinion/2020/07/21/coronavirus-swedish-herd-immunity-drove-up-death-toll-column/5472100002/ )

(I posted an earlier topic on masks at: https://www.uglyhedgehog.com/t-656094-1.html#11402347)

What I don’t understand about this post is why this forum continues to beat a dead horse on this topic of masks and their usefulness in reducing the spread of Covid-19. My county has seen a steady increase in cases over the duration of this pandemic and to my eyes the reason is plain to see - the majority of people out and about in the community in retail establishments, restaurants, and other places where the public gathers aren’t wearing masks. In my opinion, even social distancing alone in such situations is not as effective as it is said to be. If all the self-absorbed, science sceptical idiots out there would show a smidgeon of caring about their fellow citizens the growth in new infections would certainly show it.

Stan

My wife is a retired Surgical RN/OR Charge Nurse, our daughter is in Medical School, our oldest son trained as an Army Medic then changed to Civil Affairs and all three tell me the main reason for the type of mask most people are using is to keep you from coughing or sneezing on someone. They are not very good at keeping things out because of poor filtration or poor fit/seal on the face. There are better ones that you only see in infectious disease wards, labs and some OR's that do seal well, have better filters etc. All the way up to Hazmat suits.

I have an industrial/lab grade half face mask made by 3M (I have asthma and got one for a painting project in the house several years ago so I ordered a new one.) that uses filter cartridges of several types from just dust all the way up to N95 (medical standard for "standard duty") and then even further to N100 (99.9% filtration on particles .3 microns or larger) and what I got for this Covid thing - P100, it also keeps out and resists petroleum based solvents.

Add a face shield or goggles over my glasses and people sometimes freak out when they see it. But it works and is actually easier to breath in then the floppy ear loop cloth things. I wear one of those for a while and it gets damp from my breath and I feel like I am breathing through a wet towel. It also doesn't fog my glasses as the filters are mounted on each cheek so the air goes sideways or down instead of up past a poor nose fit. I ordered it in the first place because I remembered my training with Army gas masks and they were designed for people doing hard physical things and needed to breath. I figured industrial and lab workers needed the same kind of thing so I bought one.

Oh, the cartridges that hold the filters (replaceable or non-replaceable are both available) have a bayonet mount similar to a camera lens mount and these P100's that I ordered are bright magenta/pink. My wife took some pictures of me with it on and sent them to our daughter. She wanted to know where I was planning on going in that (answer-anywhere but a hazmat environment or disease lab) and how did I manage to get one. Amazon has lots (and lots of cheap Chinese ripoffs). I got 3M from an industrial safety gear supply company. The hard part was finding a size large - tons of medium and small for sale on Amazon but large took almost a full day of online searching to find. I tracked it down through a review by "The Wirecutter" (they evaluate industrial stuff) even the New York Times published their review here it is: https://www.nytimes.com/wirecutter/reviews/best-respirator-mask/
They also evaluate the other types of mask in the article. Read the whole thing. There is a lot of good and interesting information in the full article.

No one had the #1 choice 3M 6501QL (Quick Latch-to drop it while still wearing the harness so you can eat, drink, etc) in size large so I got the 7503* which uses the same cartridges but without the quick latch feature. I can afford to not eat more often anyway.

*They mention the 7502 - that is my mask in size medium.

Now if you maintain social distancing, wash or sanitize your hands, don't rub or pock your eyes the other masks help and at least make everyone feel better. (but not cheap cloth or bandannas like a western holdup man those are 99+% feel better and -1% effective)