For more than two years, Hong Kong managed to keep out the novel coronavirus. Thanks to China’s extremely strict limits on crowds and travel—the so-called “COVID Zero” approach to the pandemic—daily new cases in the city of 7.5 million rarely exceeded a hundred, even during the worst global surges.

Then came Omicron. Starting in January, the extremely transmissible new variant of the SARS-CoV-2 virus punched right through Hong Kong’s restrictions. Authorities logged a record 3,570 new infections on Monday—and a record 30 deaths on Sunday.

“We did really well in keeping COVID variants out of Hong Kong between April and December 2021, but our luck finally ran out over the new year,” Ben Cowling, a professor of epidemiology at the University of Hong Kong, told The Daily Beast. Now hospitals are filling up. Testing sites are overwhelmed. City leaders are getting desperate.

Omicron’s takeover in Hong Kong underscores the flaws in the COVID Zero strategy. Locking down entire populations delays, but doesn’t entirely prevent, viral transmission. And when the pathogen finally finds transmission pathways, there’s very little natural immunity to slow its subsequent spread.