Looking at reproduction level

The R number has gained notoriety in recent weeks for its role in providing important evidence on which politicians will base decisions on easing Scotland out of lockdown.

The reproduction value helps to provide politicians with insight on what restrictions should be in place while governments across the world battle the virus and is dictating, to an extent, when these might be lifted.

We heard from Scotland’s chief statistician Roger Halliday during a briefing to the media yesterday to explore what it all means.

What is the R number?

The reproduction level, or R number, is a measure of the average number of people that will be infected by one individual with Covid-19.

If the R number was above one it would show the virus is spreading in the population and if it is below one it is declining in the population in terms of the number of cases.

The Scottish Government’s chief statistician Roger Halliday said this should “decrease over time” as people become infected.

If 50% of the population has been infected then the R should be half of what it was at the start of the epidemic.

How is the R number measured?

This is calculated on a two-step process. Firstly, information about deaths is used with information about the progression of Covid-19 to estimate the number of infectious people.

Secondly, the day-by-day changes in the numbers of infectious people are used to estimate R.

The government’s approach draws upon publicly available information using a publicly available pyramid model from the Imperial College of London to understand how the epidemic is spreading in Scotland and its impact.

How has the R number changed over time? The Scottish Government started modelling the spread of the virus in early March and got estimates back to the end of February.

The latest estimate from May 8 shows the R number is between 0.7 and 1.

Before the stay at home public health restrictions were put in place, the R number was above one, and was calculated as being between four and six.