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.
However, there is a huge caveat to this statistic. The figures only go up to February 11 so would not capture anyone who died after this date. The first European fatality, a Chinese tourist, was admitted to hospital in France on January 25 and his death was announced on February 15 – so there can be a significant time lag between disease onset and death.
And only the most severe cases were being found at the beginning of the epidemic so there was a "huge bias" towards the most severe cases, says Dr Ryan from the WHO.
At the beginning of the swine flu epidemic the case fatality rate was around 20 per cent before plummeting once all the mild cases had been found.
He added that health staff in Wuhan were also refining their practices, for example by prioritising patients with underlying disease.
"The falling fatality rate is a mixture of having better and better interventions and finding more mild cases," he said.
What about health workers?
The study looks at the heavy toll the outbreak has taken on health workers – 1,700 have been diagnosed with the disease and five have died. It says no ‘super-spreading’ events have taken place in hospitals – that is where a single individual disproportionately infects many others – but does not clarify what it means by this. Super spreading events have been a feature of other coronavirus outbreaks.
But it also shows that fewer health workers are now being infected – this may be because infection control policies have been tightened up.
Are the number of cases still growing?
Over the last week or so the WHO has said that the number of daily cases seems to be stabilising but it has warned repeatedly that the outbreak could go in any direction. This study shows that the number of new diagnoses peaked around February 2 when there were around 3,500 new diagnoses, compared to 2,500 on February 11.
The paper puts the fall down to China’s aggressive containment policies – but it is worth remembering that the CDC is a government agency. The report states: “Isolation of whole cities, broadcast of critical information… with high frequency through multiple channels and mobilisation of multi-sector rapid response teams is helping to curb the epidemic.”
However, it also warns that the disease could “rebound”. WHO director general Dr Tedros Adhanom Ghebreyesus also urged caution when presenting the figures.
“This trend must be interpreted very cautiously. Trends can change as new populations are affected. It’s too early to tell if this reported decline will continue. Every scenario is still on the table,” he said.
Prof Gray agreed.
“As China resumes normal work and school activities ... one would anticipate that Covid-19 infections will likely increase, at least initially and eventually plateau. My concern is what will happen with the world case count if the virus becomes entrenched in other countries with large cities with dense human populations.”
Does the study shed more light on where the disease emerged?
The study says that the epidemic curve – that is, the trajectory of the outbreak – is consistent with the theory that Covid-19 began in the Huanan seafood wholesale market. There were a few cases from the same source at the beginning but then, as the disease started to spread in the human population, there were multiple new sources of infection later. But it still cannot point to the animal which first spread the disease.