# What is the R number and why is it key?

## When scientists are faced with a new epidemic, they must act to understand how quickly it spreads.

They do this by estimating the basic reproduction number called the R0, or R nought, which reflects how infectious a virus is.

What is the R0?

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It is defined as the average number of people an infected person can expect to pass the virus on to before any widespread immunity or attempts at immunisation are made.

For example, if one person develops an infection and transmits it to two other people, the number would be R2. The higher the number, the faster it progresses.

If it is greater than one, the infection will spread exponentially, but if it is lower than one, it will spread slowly and eventually die out, according to the Centre for Evidence-Based Medicine (CEBM) at Oxford University.

What is the R0 for the new coronavirus?

The World Health Organisation estimated at the start of March the R0 was between 2 and 2.5, but estimates from different studies vary widely.

According to nine studies in China and South Korea between December and March, the mean estimated R0 was 2.63, the CEBM said.

An Imperial College London study widely seen as influencing the Governmentâ€™s lockdown measures predicted around 490,000 deaths in the UK with a model using an R0 of 2.4.

What is the rate of infection now?

The R value is now thought to be between 0.5 and 1, meaning that each person infected with the virus passes it on to less than one other person.

This means the total number of cases is falling.

Strict social distancing measures, a dramatic drop in use of public transport, and people leaving the house only for essential trips are all cited as effective measures to slow transmission.

As fewer people come into contact with each other, there are fewer chances for the virus to spread.

Reducing the R value to manageable levels forms one of the Governmentâ€™s five tests it says must be reached before lockdown restrictions are adjusted.

How is the R value calculated?

One statistical method is dubbed the SIR model which accounts for three factors: the number of susceptible individuals, the number of infected people, and the rate of removal from the population, either by recovery or death.