With N95 masks, the face seal is the killer. I don't have much luck with them at all; must be my big melon. (A full-face P100 respirator is my choice for "all-in" situations.)

I wonder, though, given a perfect face seal: AFAIK, flu viruses don't travel solo, but hitchhike in droplets of moisture etc. These are much larger, and the filter medium should catch them. Ditto an infected person -- they should decrease the viral load being released.

For individuals looking for 100% protection, that may not be matter much. But across an infected population, it may (?) tip the balance in favour of a great many individuals.

All in theory, of course.

[EDIT: the two posts above have beat me to the punch.]