Coronavirus (COVID-19): modelling the epidemic

What is the reproductive rate R and how it is estimated

This infected pool of people can increase quickly or more slowly depending on the effectiveness of the social and other measures in place in Scotland. This can be described as the effective reproductive rate (rate of transmission of the virus) at a given point in time (R_t). If the R_t is above 1 the growth in the epidemic in Scotland will grow.

The basic reproduction number is affected by several factors

• The underlying infectiousness of the organism
• How long people who have Covid can infect others for
• The number of people in the population that the affected patients are in contact with, and how intense that contact is.
• In addition, R_t should decrease over time: as people become infected in a population there are fewer susceptible people left as they are either infected, have recovered, or have died. So, if 50% of the population have been infected, R is half of what it would have been at the start of the epidemic.
• If policies have the effect of reducing the number of people someone comes into contact with, that would in turn reduce R.

The R value presented in this document and in the Scottish Government's Coronavirus (COVID-19): framework for decision making, is calculated through modelling of the path of the virus, using data on cases and deaths. As such, it is an estimate with a level of uncertainty. Given the levels of uncertainty, we use modelling outputs from a number of different academic groups (mentioned above) to validate our estimate the R value for Scotland. No modelling group use exactly the same approach to modelling R_t. As such, their outputs are brought together to give a consensus view.

Importantly, particularly given the uncertainty in the R value, the Scottish Government publishes and uses a number of different pieces of data to understand the current path of the virus. This includes the number of cases, hospital and ICU bed use, and deaths.