A doctor spent all 2020, masked but unvaccinated, intubating Covid patients. And was not infected.
The mask prevents catching covid from others who have it. Works both ways.
Not perfect, but highly effective.
Remember R > 1, if we only had a vaccine just 50% effective, it would stop the spread?
N95 is at least 80% effective, and didn't take any development. Just ramping up production.
83% effective in this graphic. Which exceeds effectiveness of most of the vaccines, most of the time.
This report describes face mask or respirator effectiveness in helping protect against COVID-19 infection.
www.cdc.gov
It might not stop the pandemic (always looking for an unmasked host), but would hold it at bay as long as needed.