nothing is a perfect preventative against virus infection.
the point is to reduce the risk as much as possible and this means that masks serve a useful purpose in trapping much of the micro-droplet vector as possible, depending on how the mask is worn.
but infection inside the mask from trapping exhalations can cause problems too. bacterial growth in the moist warm medium can cause bacterial infections.
there is also a slight accumulation of CO2 inside the N type masks and these should be worn only for relatively short periods.
overall though, the mask is our first and most important - and yes, effective, line of defence.
