What the statistics from the outbreak so far can tell us about infection and mortality rates

The startling spread of the coronavirus across the globe is causing understandable alarm. But though it is still too early to draw definitive conclusions about how many deaths may occur, the statistics do point to general trends that can get lost in the drama.

At present, one thing that does seem clear is that the vast majority of people who get the disease will survive.

The World Health Organization (WHO) has estimated the mortality rate from Covid-19 is about 3.4%. That is higher than seasonal flu and is cause for concern – but even if it is correct, more than 96% of people who become infected with the coronavirus will recover.

Any death is, of course, devastating for the families involved. But it is also worth noting the WHO estimate is based on confirmed infections and deaths, meaning it does not take into account mild cases that may not be diagnosed – cases that would lower the mortality rate.

Indeed experts say that, in reality, the mortality rate is likely to be nearer to 1% or less. In other words, more than 99% of those who become infected are expected to survive.

And not everyone will become infected. According to the chief medical officer for England, Prof Chris Whitty, the worst-case scenario is that about 80% of the UK population becomes infected. But that does not necessarily mean there will be 500,000 deaths – as would be expected from a 1% mortality rate.

One reason is because the 80% figure is an estimate. “That is an entirely speculative number,” Whitty told the press conference, adding that it was not known how many people would ultimately become infected.

Another factor that will affect the number of deaths, and hence mortality rate, is who gets infected. According to figures from the Chinese Centers for Disease Control and Prevention, those aged 10-39 have a 0.2% mortality rate, while those in their 60s have a mortality rate of 3.6%. For those aged 80 and older this rises to almost 15%.

While such figures may seem alarming, particularly for the elderly, it means that if the spread of Covid-19 to the most vulnerable populations can be prevented, or at least limited, there will be far fewer deaths than may otherwise be expected.

In other words, protecting the vulnerable, including those with underlying health conditions, is crucial.

Experts currently caution against reading too much into figures from the UK. On Friday morning there were 798 confirmed cases of Covid-19 in the UK, and 10 deaths – although the prime minister, Boris Johnson, has suggested as many as 10,000 people in the UK could be already infected with the virus.

But simply looking at the current snapshot can be misleading – because of the time lag between infection, disease and death, those who have already died would have been infected earlier, when the number of cases were lower, while there has also been a rapid uptick in new cases.

“It is still technically early [in the outbreak]. You are not quite sure if the cases and the deaths are representative of what will pan out in the long term,” said Dr Michael Head, senior research fellow in global health, University of Southampton. “As we go forward we will basically have better data,” he added.

One thing that is clear is that mortality rates do differ from country to country. At present, Italy has been estimated to have a mortality rate of more than 6%, while in South Korea the mortality rate has been estimated at below 1%.

It is difficult to unpick exactly what is behind such differences. However a number of factors may be at play, including how many people are being tested: aggressive testing may flag up milder cases that would otherwise have been missed, meaning the crude mortality rate is lower.

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Indeed it is believed that the coronavirus may have been circulating unnoticed in northern Italy since mid-January, while it took repeated hospital visits before a 38-year-old man with symptoms was diagnosed with Covid-19.

Other factors include the type of public health measures being employed, and the demographics of the population. Italy has a large elderly population – about 23% of the population is over the age of 65 – and it is known that older people are at greater risk from Covid-19.

In short, Boris Johnson is correct to say that the death toll in the UK will rise. But at this point in time, it is not possible to put a figure on just how many will die. If public health measures are successful, there is hope the vast majority will be spared.