There's quite a few articles about how South Korea has handled this. They give credit to mass testing early on, strict quarantine and tracking of infected people. Masks are not mentioned anywhere, because they've got nothing to do with the sucess.
The backbone of Korea’s success has been mass, indiscriminate testing, followed by rigorous contact tracing and the quarantine of anyone the carrier has come into contact with. As of March 19, the country has conducted more than 307,000 tests, the highest per capita in the world. The UK has conducted 64,600; The US even less that. “You have countries like the US right now, where there's a fairly strict criteria of who can be tested,” says Kee Park, a lecturer on global health at Harvard Medical School. “I know people personally who have symptoms that are highly suspicious, but they don't meet all the criteria and so they're not being tested.”
“[South Korea’s] extensive testing is a very valuable tool to both control the virus and understand and measure the effectiveness of the responses that are taking place,” says Michael Mina, assistant professor at the Center for Communicable Disease Dynamics at Harvard University. “It’s allowed individuals to take matters into their own hands and make social distancing decisions on their own, both to protect those around them and to protect themselves from those who are infected around them.”
To carry out testing at this scale requires extraordinary coordination. The Wall Street Journal reports that the country can test more than 20,000 people a day at 633 testing sites nationwide. A smartphone app provides GPS maps to track the infection’s spread. Medics pitch massive white tents on roadsides, where citizens receive free drive through testing, reducing the need to clean infected hazmat suits. Results are swift, too, coming by text within 24 hours.
The response has melded with Korean technological ingenuity, explains Park.“Koreans are super good at making things convenient for people – we don't have any patience,” he says. “South Korea is one of the most wired countries in the world, where everybody uses cell phones for just about everything, and [the government] was able to use our cell phones to not only track but send warnings, like ‘watch out, there's a Covid-19 patient in your vicinity.’”
https://www.wired.co.uk/article/south-korea-coronavirus
If you think about it, a non-airtight bit of breathable cloth is not going to stop particles as small as a virus. They're designed to stop surgeons dribbling onto patients!
The virus doesn't appear to be very well aerosolised (nuclei of <=5
μm), meaning it doesn't hang around in the air waiting to be inhaled.
C19 seems to mostly be transmitted through droplets (>5
μm ) which land on surfaces and are then introduced to the face via people's hands.
If I had to guess, I'd think that regular face masks are mostly useful when worn by already infected people to stop them from coughing droplets everywhere. But they're likely also helpful for medical professionals who have a lot of close contact with infected individuals, and might otherwise breathe in a stray droplet or two. In either case, a properly fitted breathable cloth should do the trick (N95 can stop 95% of particles >0.3
μm).
Probably not of great use to the average Joe going about his day in the toilet paper aisle of the supermarket, though. Social distancing is going to be more effective in that circumstance.
(Obviously, the science isn't fully in yet, so take with a heap of salt).