It is one of the many mysteries of the coronavirus pandemic: Why has the death toll from covid-19 apparently been lower in Asia than in Western Europe and North America?

Even allowing for different testing policies and counting methods, and questions over full disclosure of cases, stark differences in mortality across the world have caught the attention of researchers trying to crack the coronavirus code.

Parts of Asia reacted quickly to the threat and largely started social distancing earlier on. But researchers are also examining other factors, including differences in genetics and immune system responses, separate virus strains and regional contrasts in obesity levels and general health.

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China, where the virus emerged late last year in Wuhan, has recorded fewer than 5,000 deaths, which translates to three deaths per million inhabitants. Japan has around seven per million, Pakistan six, South Korea and Indonesia five, India three and Thailand fewer than one per million. Vietnam, Cambodia and Mongolia say they have recorded zero covid-19-related deaths.

Compare that with about 100 deaths per million in Germany, about 180 in Canada, nearly 300 in the United States and more than 500 in Britain, Italy and Spain. Scientists at Japan’s Chiba University plotted the trajectory of the virus across the world and said they noticed stark regional disparities.