Read or download the full COVID-19 briefing paper: what is the R number?
Reproduction number (or R) is one of the most important numbers of the coronavirus pandemic. So what is it? And how is it estimated?
NB: information contained in this briefing paper was accurate at the time of publication, 15 May 2020.
What is the ‘R’ number?
The ‘R’ number is a number that says how many people each person who has a disease will infect. The ‘R’ number can change, depending on how many contacts people have, such as during an epidemic lockdown. The number is always an average. It is the rate of new people infected when it is passed on.
Dr Sebastian Funk, Associate Professor and Director of the Centre for the Mathematical modelling of infectious diseases, explains the R number on the BBC Radio 4 Today Programme, Friday 15 May (at 48 minutes 41 seconds).
Why is the ‘R’ number important?
Put simply, as long as the ‘R’ number is less than 1, every person with COVID-19 will infect less than one person so the outbreak will decrease in size until it ends. However, if the ‘R’ was to go back above 1, the number of cases could increase, potentially leading to a further outbreak. If ‘R’ is more than one the epidemic is increasing, if less than 1, the epidemic is shrinking. Before we went into the lockdown in March 2020, experts said it was possibly 2.6, but now with easing of the lockdown it is below 1.
There are complexities around this, with some types of outbreak putting the ‘R’ number up while being easier to control, such as those in care homes, and smaller outbreaks having a larger effect on the ‘R’ number when there are only a small number of infections1.
One problem in measuring the infections in this way is that it only gives a view of the past.
Jenny Harries (Deputy Chief Medical Officer for England) said:
"R is a very standard way of looking and comparing what's happening and it's a very important measure but the real outcome that we're looking for is a reduction in the number of cases and getting rid of the epidemic. That is the focus, not R. R is a representation of what is happening in that fight." - (Sky News, 15 May 2020).
Read or download the full COVID-19 briefing paper: what is the R number?
