D URING THE first two years of the covid-19 pandemic, rich Asian countries had lower case rates than almost anywhere else. But like a dam holding back a flood, the longer a region’s defences hold up, the worse the resulting disaster once they fail.

The Omicron variant has swept remarkably fast through places with little past exposure to covid. As a share of population, South Korea has logged more cases in 2022 than America has during the entire pandemic. Hong Kong’s surge has been even more abrupt. Until this February, it barely had any positive tests. Its cumulative caseload per person now matches Finland’s.

In Europe Omicron has caused greater increases in cases than in hospitalisations or deaths. Two factors have combined to yield such mild disease: strong immunity levels and Omicron’s low severity when compared with Delta. Until recently, it was unclear which mattered more. But the Asian data show that immunity is crucial, and that Omicron is still deadly for unvaccinated people who have not yet had covid.

Hong Kong and South Korea provide a natural experiment. Both were mostly covid-free before 2022, and had little infection-induced immunity. But South Korea has had a strong vaccine roll-out, whereas Hong Kong has had one of the rich world’s worst. Its messaging was decidedly mixed, giving equal weight to arguments for and against vaccination, and telling people with chronic diseases to consult doctors before getting jabs. Its nurses worried about personal liability for mishaps.

Hong Kong also made heavy use of Sinovac, a Chinese vaccine that is less effective than Western shots. According to Ming Pao, a newspaper, of Hong Kongers who died in the recent wave, 71% were unvaccinated and another 25% had received Sinovac. Just 4% got Western vaccines.

Unfortunately, Hong Kong’s elderly were unusually hesitant. In South Korea take-up of booster shots, which are needed to protect against Omicron, is greatest among the old. In Hong Kong, in contrast, those most likely to get boosted are middle-aged. Just 37% of its septuagenarians and 14% of those 80 or older, the most vulnerable groups, are boosted—similar to the shares among people aged 20-39 and teenagers, respectively. When Omicron first struck, these rates were even lower.

This discrepancy has had a big impact. During the Omicron wave, Hong Kong’s official covid mortality rate has been five times higher than South Korea’s. These figures can be biased by differences in testing rates and judgments on causes of death. But fair comparisons can be made with excess mortality—the gap between the number of deaths from all causes and the number expected under normal conditions.

Although recent total-mortality numbers have not been published, our best estimate suggests that since February 1st 2.4 times as many people have died because of the pandemic in Hong Kong as in South Korea. At the peak of Hong Kong’s surge in early March, its daily excess-death rate was between those registered in London (1.8 per 100,000 people) and New York (5.1) when covid first struck—a period with no vaccines, little testing and an incorrect understanding of how the virus spreads. ■

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Sources: Our World in Data; Public Health England; CDC;Korea Disease Control and Prevention Agency; Hong Kong Department of Health; The Economist