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The figure at the root of so much global angst about coronavirus is currently 4.7 per cent.

That is the proportion of people, as of Sunday afternoon, who have died after being diagnosed with the virus — 32,137 out of the 685,623 who have tested positive for Covid-19 around the world.

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It compares with a death rate of around 0.1 per cent for seasonal flu and 0.2 per cent for pneumonia in high-income countries. However, 4.7 per cent is not only changeable but frustratingly unreliable, both for governments seeking to calibrate their policy response and for citizens trying to gauge how much they should worry.

The proportion of people who have died from the disease varies strikingly from country to country. Researchers warn that there are so many uncertainties — not least over the true number of infections — that it remains almost impossible to draw firm conclusions about the death rate.

Mike Ryan, executive director of the World Health Organization’s health emergencies programme, has outlined four factors that might contribute to the differing mortality rates: who becomes infected, what stage the epidemic has reached in a country, how much testing a country is doing, and how well different healthcare systems are coping.