Scott Alexander thinks that ivermectin might work in places where people have a lot of worm infections: https://astralcodexten.substack.com/p/ivermectin-much-more-t...

Somehow I start to suspect that masks might have a similar effectiveness that are dependant on climate, air humidity and widespread co-infections. Maybe not effective against covid but might prevent other infections that ultimately make covid worse or make them more susceptible to covid.