Coronavirus Q&A: What is the R value?

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?

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 (WHO) 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 is 2.63, the CEBM said.

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

By comparison, measles has an estimated value of more than R14 while smallpox is almost R6, the CEBM said.

– What is the rate of infection now?

Earlier this month, Foreign Secretary Dominic Raab said it was “almost certainly below one in the community”, as he announced at least three more weeks of lockdown.

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 at slowing transmission.

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

“But overall we still don’t have the infection rate down as far as we need to”, Mr Raab said, warning that lifting restrictions now would risk a dangerous second peak of Covid-19.

– How is the R value calculated?

There are many different mathematical and statistical models used to predict the R value.

One 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.

But estimates between studies vary widely owing to the quality and reliability of data and different types of epidemiological models, the CEBM said.

The outcome value is affected by the proportion of susceptible people, the density of populations, the infectiousness of the organism, and the rate of removal of cases either by recovery or death, it added.