The N95 masks don't prevent one from getting or transmitting viruses, they greatly decrease the viral load (the amount of virus that is "shared"). P100 respirators are used in isolation wards because they block 100% of the viral load (as opposed to ~95% for N95 and KN95 masks) but those are cumbersome (I have one for work). N95 masks and the vaccine work by decreasing the viral load that your immune system has to fight. They work if the covid is less severe than if you hadn't worn a mask or been vaccinated. We are used to vaccines and other preventatives being 100% effective, and the masks and mRNA vaccines work if they stop OR to decrease the spread or severity of diseases.