Is no guarantee that you're preventing a symptomatic carriers from entering. But immunologically and biologically speaking, people who have been vaccinated against any specific disease are less likely to have or carry that disease. That reality is about as generally accepted as one could be.

So at least those countries that maintain that requirement of completing vaccination series have some of a public health leg to stand on....even if it's not foolproof because if somebody asymptomatically transmits covid they're not introducing it to a place where it hasn't been. But it's a much more practical position to take than requiring a negative covid test.

Requiring a test - when the entire focus of the vaccine was to get people vaccinated so that normalcy could resume - is just plain stupid at this point.