Question to the Department of Health and Social Care:

To ask the Secretary of State for Health and Social Care, what assessment he makes of the accuracy of previous R estimates for the rate of covid-19 transmission when actual transmission rates become measurable.

The reproduction number (R) is the average number of secondary infections produced by a single infected person.

‘R’ is an average value that can vary in different parts of the country, communities, and subsections of the population. It cannot be measured directly and is estimated based on data such as numbers of hospitalisations and deaths. There is always uncertainty around its exact value. This becomes even more of a problem when calculating ‘R’ using small numbers of cases, either due to lower infection rates or smaller geographical areas.

The Government Office for Science currently publishes the latest estimate of ‘R’ for the United Kingdom and NHS England regions on a weekly basis. ‘R’ is estimated by a number of independent modelling groups based in universities and Public Health England (PHE). The modelling groups discuss their individual estimates at the Science Pandemic Influenza Modelling group (SPI-M) – a subgroup of the Scientific Advisory Group for Emergencies (SAGE). Attendees compare the different estimates and SPI-M collectively agrees a range which ‘R’ is very likely to be within, which is then reviewed and endorsed by SAGE. As part of this weekly review by SPI-M and SAGE, estimates of ‘R’ are considered alongside other metrics, such as estimates of prevalence and incidence from the Office of National Statistics COVID-19 Infection Survey, or trends from Pillar 2 testing.