I remember reading it in a comment on HN and I'm struggling to find the source. It was about improvised woven fabric masks, not medical type masks. But even if

I remember reading it in a comment on HN and I'm struggling to find the source. It was about improvised woven fabric masks, not medical type masks. But even if this is actually false, it doesn't change the fact that reducing flow rate does not "by definition" reduce virus spread (assuming this means infection spread, and not a literal synonym). Infection risk depends on more than just virus particle count, so you need to count infections (and as many other comments point out, that is being done, and the number of infections is reduced).