Doesn't stop but reduces a lot. You are about 10x less likely to catch covid with a vaccine and about 2x less likely to spread it if you do.
Also if you compare a population of 100 million people who mostly have that 20x protection with one that doesn't and infect one person the the end result is one or two infections in the first case and about 100 million in the second.
They virus "survives" much shorter time in a vaccinated body hence less chance of tranmission to a new host.
That time is however not zero.
Furthermore, you can carry the virus outside your body.
I'm looking for an educated answer, I'm not anti-vaxx, far from it, I'm just curious about it.