Coronavirus: What is the R number and how is it calculated?

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There is a simple but crucial number at the heart of understanding the threat posed by the coronavirus. It's guiding governments around the world on the actions needed to save lives and to lift lockdown.

It is called the reproduction number, or simply the R value.

What is R?

The reproduction number is a way of rating coronavirus or any disease's ability to spread.

It's the number of people that one infected person will pass a virus on to, on average.

Measles has one of the highest numbers with an R number of 15 in populations without immunity.

That means, on average, one person will spread measles to 15 others.

Coronavirus (Covid-19) - known officially as Sars-CoV-2 - has a reproduction number of about three, but estimates vary.

How is R calculated?

You can't capture the moment people are infected. Instead, scientists work backwards.

Data - such as the number of people dying, admitted to hospital or testing positive for the virus - is used to estimate how easily the virus is spreading.

Why is a number above one dangerous?

If the reproduction number is higher than one, then the number of cases increases very fast - it snowballs like debt on an unpaid credit card.

But if the number is lower the disease will eventually stop spreading, as not enough new people are being infected to sustain the outbreak.

Governments everywhere want to force the reproduction number down from about three (the R number if we took no action) to below one.

What is the R number in the UK?

The reproduction number is not fixed. Instead, it changes as our behaviour changes or as immunity develops.

Mathematical modellers at Imperial College London attempted to track how the number changed as isolation, social distancing and the full lockdown were introduced in the spring.

Before any measures came in, the number was well above one and the conditions were ripe for a large outbreak. Successive restrictions brought it down, but it was not until full lockdown that it was driven below one.

Across the UK at the start of September, the R number was estimated to be between 0.9 and 1.1.

One member of the government's Scientific Advisory Group for Emergencies (Sage), Prof John Edmunds, told ITV's Robert Peston the number had risen "above one", and the UK was in a "risky period".

Are there regional differences?

Yes - the highest R rate is currently estimated to be in Northern Ireland, at between 1.1 and 1.6. Rules on social distancing and meeting up with people you don't live with have recently been tightened there once again.

Wales is estimated to have the lowest rate, between 0.5 and 0.9.

But even so, the spread of the virus is still troubling officials in the Caerphilly area of South Wales - and a local lockdown is being introduced.

What does R mean for tightening or easing lockdown?

The UK government said in the past that the R number was one of the most important factors in making policy decisions.

It then said estimates were less reliable and less useful because the number of cases was relatively low.

But with cases on the rise again, more attention will be paid to local and national R rates.

The aim will be to keep the reproduction number below one.

Is it the most important number?

The reproduction number is one of the big three.

Another is severity - some people have a very mild disease that does not cause many problems. But coronavirus and the disease it causes, Covid-19, can be severe and deadly.

The last is the number of cases, which is important for deciding when to act. If you have a high number, but ease restrictions so the reproduction number is about one, then you will continue to have a high number of cases.

What about a vaccine?

Having a vaccine is another way to bring down the reproduction number.

A coronavirus patient would naturally infect three others on average, but if a vaccine could protect two of them from infection, then the reproduction number would fall from three to one.

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