They wore medical grade mask in the first place. Hygiene and cautious about it. They stopped and block visits from China ( or any where else with COVID ) very early on. Instead of acting everything is going to be great despite all the warnings, they were mindful and kept social gathering to minimal. ( Comparatively speaking ). Citizens were also proactively reporting if they were having symptoms, and government are testing every case as well as tracing where they have been and issue warning.

Medical Mask also meant even though the case broke out in a city with very high population density, the spread is far lower ( again comparatively speaking ).