The new technique is called contact tracing with mandatory testing.What? Wait? They only quarantined the infected people? How does that work? Is this some new technique? Why wouldn't they quarantine the healthy people?
Contact tracing a boat is easier than contact tracing a city. And, as part of their ticket purchase, the passengers agree to be tested. No test, no ticket.
If you want to give up enough freedom to allow real contact tracing in your city, complete with mandatory testing, then cities can do this, too. Think Seoul.
If you don’t like the idea of mandatory contact tracing with mandatory testing, then maybe this idea isn’t for you.