What started Hong Kong's third Covid-19 wave?

For months, Hong Kong was seen as an example of how a community could successfully contain Covid-19. The city was quick to isolate and trace infections and implemented measures to keep the public safe, moves which kept the number of cases relatively low. But after three weeks of zero local infections, Covid-19 cases in the city started to soar and Hong Kong entered its "third wave." So what happened to change things so suddenly?

(Photo: SCMP / Edmond So)