Hello and welcome to more or less on the BBC World Service,

where your ever curious guide to the numbers all around us, and I'm Tim Halford.

The question of just how dangerous the new coronavirus really is is absolutely crucial.

If the fatality rate is high, there could be dreadful consequences if we relax the lockdowns

that have been imposed across much of the world. If it's not so high, from any countries,

the worst might already be behind us, not just of this first wave, but of the entire pandemic.

But the frustrating thing is, we're still not sure. On the face of it, the virus is terrifying.

The number of confirmed cases worldwide is more than 3 million, of whom 250,000 people have died.

That suggests that the fatality rate is 7%, a huge number. If the virus spread everywhere,

it might eventually kill hundreds of millions of people.

Fortunately, the virus isn't nearly as dangerous as that 7% number makes it seem.

That number is called the case fatality rate, and it's not a reliable indicator.

I've been talking about this to Carl Hennegan, who's both a practicing medical doctor

and a professor of evidence-based medicine at the University of Oxford.

If you actually look at the case fatality rate today, you'd see that the UK was at the top,

and it's just above 15%, which gets really worrying. But actually, what we found really

is that measure is not very helpful.

Yes, the UK has the highest case fatality rate in the world, but that's because the UK was

slow to administer a widespread testing programme. Instead, the country has been testing people

who are in hospital, that is, the most severe cases. Countries with much more testing will count more

of those mild or asymptomatic cases, and so they'll have a lower measured case fatality rate.

A good example of this difference would be if you looked at South Korea, where they've done

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