So 2 in 5 wearing them incorrectly versus nobody wearing them at all? Which is better?
I think at that point any benefit from an epidemiology point of view is wiped out due to an increase in risky human behaviour.
I was reading about a drug called PrEP which, when taken, basically prevents HIV from being caught. It's prescribed to gay/bisexual men. It works near enough 100% of the time. A proper miracle drug when you think about it, mad how it's not front page news.
Anyway, what happened was those given it started 'barebacking' because the HIV risk was effectively gone... and of course that meant in the six months immediately after starting treatment there was a 25% to 50% increase in every other STD in those using PrEP. Because risky behaviour was encouraged.
Masks do the same thing; they are a visual cue to people that they're 'safe', even though they aren't, so they'll do things they wouldn't otherwise do. That's why the mask mandate in Wales is doing basically nothing, because any positive effect is cancelled out.