How bad is the Covid-19 coronavirus outbreak likely to get?

By Debora MacKenzie

TPG/Zuma Press/PA Images

The World Health Organization has now named the new coronavirus disease: Covid-19.

If the virus isnt halted, it could infect 60 per cent of the worlds population and kill one in 100 of those infected around 50 million people Gabriel Leung, at the University of Hong Kong, told TheGuardian on 11 February.

But no one knows if it really will,because we dont know whether the virus can be contained, how deadly it is andhow many people have it.

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The number of confirmed casesglobally reached 42,000 on Tuesday, but the rise in cases has been slowing since 6 February. This suggests Chinas decision to limit peoples movements in the most affected province, Hubei, isworking and that containment may be effective.

That isnt certain, however. Thedecline may also reflect overwhelmed hospitals or testing labs. Studies continue to estimate that there are far more cases in China than those reported. Whats more, tests of people repatriated from China hint there are many mild and asymptomatic cases, who may be able to spread the virus but arent necessarily being tested or quarantined.

Even if mild cases are being tested, they may not have been making it into official figures. Diagnostic guidelines issued last week in China say people without symptoms who test positive for the virus as part of efforts to trace contacts of known cases should only be counted as confirmed cases if they start showing symptoms. The WHO said on Tuesday this would change.

As for death rates, these are hard to calculate early in an epidemic, when the outcome of most cases is still unknown, says Neil Ferguson at Imperial College London.

Using models based on the rate of rise of deaths, Ferguson and his colleagues have calculated thatsome 18 per cent of people confirmed to have the virus in the Chinese city of Wuhan die. This is similar to earlier estimates.

However, to get tested in Wuhan because of illness, not as part of contact tracing, you must have pneumonia or worse. This means death rates among confirmed cases in Wuhan are likely to be higher than among groups that include milder illness.

For example, travellers from China are tested if they have flu-like symptoms, so for them Fergusons model gives a lower death rate of 5 per cent.

Meanwhile, hints of how many people in Wuhan may really be infected with the virus come from tests of the 750 people who have been repatriated from the city to Germany and Japan. Of these, 10infections were found. We know details of eight of these cases, of which five were symptomless. This suggests that 1.3 per cent of people in Wuhan may have the virus, many unknowingly.

Based on this, Fergusons team calculates that, by 31 January, there were at least 24,000 new cases a day in Wuhan, which calls into question the current fall in case reports, which number around 3000 a day. This could also mean that total case numbers in China may now be as many as a million. If this is the case, and if all deaths in Wuhan are being detected, then, says the team, the overall death rate is only around 1per cent which matches Leungs prediction.

But age matters. In China, 80percent of deaths have been inpeople aged over 60. While China has a median age of 37.4, themedian agein the European Union, for example, is close to 43, meaning more residents may be vulnerable there.