The problem with COVID-19 all along has been that it is transmissible before being symptomatic. That is why it spread like wildfire when SARS and MERS did not. Listened to a good interview on NPR some months ago with a leading coronavirus expert who went over this in detail, but unfortunately can't remember her name or find the interview right now.

In any case, vaccines don't really change the equation here. Testing for temperature and symptoms with COVID have proven to be ineffective because asymptomatic transmission in the early stages is a thing with COVID-19 even without vaccination.