Vaccine, infection, and spreading COVID

[LSLGuy]

I’ve reported this thread for possible merger with this slightly older thread on essentially the same topic:

A couple of vaccination questions

I keep reading that we don’t know whether or not people who get vaccinated can spread the virus. Is it that hard to determine whether people are shedding the virus? Once I’ve had both doses of the vaccine and a few weeks have elapsed, how will I know whether or not I’m in the tiny percentage for whom the vaccine is not effective?