Transmission of disease. This is a "noble lie" they are perpetuating to keep vaccinated people still wearing masks and social distancing to prevent non-vaccinated people from giving up mask wearing and social distancing (also because there is no reliable way yet to verify if someone is vaccinated or not and would create too much of a double standard). This is the same reason why they said masks didn't work in feb and march and then changed their minds later on (of course they knew that masks worked, they are helpful to some degree in all respiratory illnesses - this was a master plan to prevent panic and a run on masks as they wanted medical facilities to be able to stockpile them first).
I predict that sometime around July when they have the ability to give most people the vaccine (not just the high risk groups), the health experts will suddenly change their minds and this will be the greatest vaccine in history and will definitely prevent transmission and if you are vaccinated, you no longer have to mask or social distance. This is the carrot that will be used to convince all the "I'm not anti vax, but...." people to get the vaccine.