maybe some organisation did a study, because two people sneezed close to me the first time I wore the coffee filter. I didn't get sick. Fabulous proof.
I have reasonable confidence in the filter paper itself, but the adjustment on the face is just bad. And notice the gap in adhesive tape where the paper crosses the beard, because beards are known to stop viruses, of course. The eyes, a supposed entry path for Covid-19, are not protected at all. That's about as bad as home-made fabric masks mandatory in the Czech republic.
In epidemics where the chances (of the general population, not of medics!) of meeting a sick person are moderate, and the chances of being infected are lower, you don't need a perfect protection. Cutting the spread rate by half can mean enough intensive care units for all badly sick patients, and this makes the difference. This seems to almost explain the low mortality in Austria, Germany (up to now) and Luxembourg as compared with Italy, Spain and France - Britain is next, on the wrong side.