For two years, Hong Kong successfully insulated most of its residents from Covid-19 and often went months without a single locally spread case. Then the Omicron variant showed up.

The fast-spreading mutation breached Hong Kong’s defences and has been spreading rapidly through one of the world’s most densely populated places, overflowing hospitals and isolation wards and prompting measures to test the entire 7.4 million population and hastily build six isolation and treatment centres.

The surge shows what happens when Covid-19 strikes a population unprotected by immunity from previous infections and has exposed a low vaccination rate among elderly citizens who are bearing the brunt of the crisis.

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