Reproduction Number

What is the R number?

The UK government has started publishing an interval estimate of R. This is also known as the reproduction number.

The article looks at the definition of the reproduction number. I also discuss estimation methods.

What is the reproduction number?

The reproduction number is the average number of direct infections from one case. This is over the whole time whilst people are infectious.

If the reproduction number is 2, we would expect 100 infected people to infect 200 more people. If the reproduction number is 0.5, the average group of 100 infected people infects 50 more.

Each infected people passes it onto more people, leading to exponential growth. (Image: Stanford University)

The reproduction number can change over time. If people reduce their contact with others, a virus has fewer transmissions.

There is also the basic reproduction number. This number is often labelled as R₀ (‘R-zero’). The basic reproduction number is for when the population has no immunity. When the virus is novel, people do not have immunity.

The basic reproduction number is not a biological constant. The same virus may spread in different populations at different paces. By itself, this number does not determine how fast a virus spreads. The number of initial ‘seed’ cases is important. We also need to understand…