China’s Centre for Disease Control and Prevention has released the largest study yet of patients confirmed to be carrying the coronavirus. The data from around 44,000 patients – from December 8 when the first cases emerged in Wuhan, Hubei province, up to February 11 – sheds important new light on the disease, showing who is most affected, how serious it is and how it has progressed over time. Here, we examine the data to find out what it can tell us about the new coronavirus. Who is getting the disease? Previous, smaller-scale studies have shown that older people are more likely to get the disease but younger poeple are not immune. Nearly 15,000 of those infected were aged 30 to 49 (36 per cent of the total).

The good news is that children and teenagers are not badly affected by the disease – just two per cent of those who contracted the virus (just under 1,000 children and young people) were aged 0 to 19. Men are also more likely to get the disease than women – 51.4 per cent of those who contracted Covid-19 were men. While this is not a huge difference the disease appears to be more serious in men than women.

How serious is the coronavirus? The study categorises cases as either mild, severe or critical. Mild cases included patients either without pneumonia or with only mild pneumonia. Severe cases include patients whose symptoms include shortness of breath and rapid breathing. Critical cases were those who had respiratory failure, septic shock and/or multiple organ failure. The figures show what many experts have been saying – 80 per cent of cases are mild, nearly 14 per cent are severe and nearly five per cent are critical. However, some people may wonder whether contracting pneumonia and being hospitalised constitutes a mild case. The study does not break down disease severity information into age profiles, however it is probable that older patients would have the most severe form of the disease as this would fit with death rates. Greg Gray, professor of infectious diseases at Duke University, said it was “reassuring” to see a high percentage of mild infections. But he added that the case definition for Covid-19 has changed and testing has not always been available throughout the epidemic. “Some mild infections and some severe infections and deaths from Covid-19 were likely missed. It will be good to see if the high prevalence of mild infections and low mortality rates remain stable as more patients are studied,” he said. How deadly is the disease? Out of the 44,000 confirmed cases 1,023 people have died, showing a death rate of 2.3 per cent. This is higher than for other respiratory illness pandemics – H1N1 or swine flu raced around the world in 2009 and is thought to have infected around a fifth of the global population. However, the death rate was around 0.02 per cent. Many experts believe that the overall death rate for Covid-19 will turn out to be much lower as people with very mild infections have still not been counted in the case numbers. Mathematical modellers at Imperial College, London believe that only one in 10 cases of the disease are being recorded in China. But when you examine the data more closely interesting patterns emerge. First the death rate is higher in Hubei province than in the rest of China – 2.9 per cent compared to 0.4 per cent. This might be down to the fact that the health system in the province has struggled to cope with the high number of cases.

"Lessons being learned in Wuhan are being applied elsewhere so this could explain the lower death rate outside Hubei," said WHO's director of emergencies Dr Mike Ryan. Secondly, as would be expected, the death rate is higher among older people. For those aged 80 or over it is 14.8 per cent. These numbers fall quite quite rapidly as the age groups go down. Men are also more likely to die from the disease – 63 per cent of all the deaths have been in men. The study does not discuss why this is although some experts believe that high smoking rates among men in China – around 52 per cent – could be one reason for the disparity. Another interesting statistic is how the death rate has fallen as the outbreak has progressed. For those who contracted the coronavirus before December 31 – when the first official notification of this new disease was made – the death rate was 14.4 per cent. It then went up to 15.6 per cent for those who became ill between January 1 and and 10 but then fell away quite rapidly to 0.8 per cent for those who contracted the disease after February 1.