the way it seems to work...

a vaccinated human exposed to virus, it enters blood stream, enters cells and looks like the mrna influenced cells and your b memory cells call out t cells made from vaccine processing.

you get some c19 replication but t cell eat up the cells that make the new viruses....

what walensky said is possible but very unlikely ie that a vaccinated person could make a viral load that is transmissible. that is more unlikely than a mask doing any of what they say they do!