[Nokinside]

That's true but it's very hard to keep the system 100% tight. Governments can't prevent human trade or drug trade either.

Just something like 1 infected entering per day or week means that you must keep up suppressing and tracing.

[PeterisP]

Sure, but keeping up testing, contact tracing and other South Korea-style suppression measures is no big deal compared to the severe restrictions most other countries have had to implement. It's very troubling to shut down the economy for a whole year, but it's certainly plausible to sustain those measures until a vaccine is ready.