One of those countries has been able to trace all cases in the last few months and makes extensive use of genomic sequencing. The leaks have all come out of hotel quarantine breaches. I believe that some Australian states that do not receive overseas travelers have not had a single case in months.

It may (or may not) not be feasible for bigger and non-island countries, although I suspect that the border closures will happen far more swiftly if there's another pandemic in our lifetimes.