Why calculating the coronavirus death rate is not as easy it seems

Analysis: Health correspondent Shaun Lintern explains why the latest statistics about Covid-19 mortality rates need to be treated with caution

The virus is significantly more deadly than seasonal flu but likely to be less than estimates, according to experts. (AP)

The World Health Organisation sparked a flurry of speculation on Wednesday after its latest analysis of the global coronavirus outbreak suggested the death rate from the disease was much higher than previously thought.

In a potentially worrying escalation, WHO director general Tedros Adhanom Ghebreyesus said that globally around 3.4 per cent of Covid-19 cases have died. It had previously been estimated at 2 per cent or below.

So what should we take from this new figure? Is it cause to panic and worry further? Well on one hand it is serious. If the 3.4 per cent figure is correct that is far more deadly than flu for example, which kills less than 1 per cent of those infected.

But calculating mortality rates require full information on the numbers infected and it is almost certain at this early stage that we don’t know the full picture. The WHO accepts there is still a lot that is unknown about the Covid-19 virus.

Experts were quick to point out that while you can say how many people have died, the point at which they die and the total number of infected is difficult to really know.

Coronavirus has a long incubation so many people may have the disease but not be counted in the figures. If they were, the mortality rate would be lower as the numbers infected but not dead would grow.

This is a likely position given some countries such as the US and Iran have had only limited testing and the fact that the vast majority will only experience mild symptoms. It is only a small minority that have actually had more severe issues

Prof John Edmunds, from the Centre for the Mathematical Modelling of Infectious Diseases, at the London School of Hygiene & Tropical Medicine, said it well: “What you can safely say is that if you divide the number of reported deaths by the number of reported cases you will almost certainly get the wrong answer.”

Dr Jennifer Cole from the University of London added that it was likely the fatality rates would fall with time as more people are diagnosed and tested.

It is also difficult to know how comparable the early cases in China are comparable to those in western countries where hospitals may be able to provide better care.

Most experts agree the true mortality rate may not be known for some time but is likely to be lower than 3.4 per cent with between 1 and 2 per cent being a reasonable view.

This is still a serious infection and such a mortality rate, if widespread throughout the population, would mean significant strain on the NHS.

This is why the government is taking it so seriously.