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

numbers all around us in the news and in life.

I'm Tim Hartford.

Recently, new rules started to require everyone arriving in the UK to self-isolate for 14

days.

The British government says putting travellers into quarantine will help prevent a second

wave of coronavirus.

But as the UK currently has almost a tenth of the world's total death toll, with more

than 40,000 deaths officially recorded, aren't people entering the UK or returning home

to the country less likely to be infected than the people who are already there?

Well Ruth Alexander has been looking at what the numbers can tell us, Hello Ruth.

Hi Tim.

Yes, the best way to look at this is to look at the prevalence of the disease.

How many people have the disease right now?

Well we can't observe that directly.

We have to estimate it, which is tricky.

But not too tricky for Dr Tim Russell.

He's a research fellow and mathematical modeler at the London School of Hygiene and Tropical

Medicine and he's been sharing his data with me.

Now the biggest difficulty in working out how many people are infected with COVID-19

right now is that while a few countries are finding and reporting the majority of their

cases, many countries are missing most of theirs.

...