Detailed guide: The R value and growth rate in the UK

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Latest R and growth rate

An R value between 0.7 and 0.9 means that, on average, every 10 people infected will infect between 7 and 9 other people.

A growth rate of between -5% and -2% means that the number of new infections is shrinking by between 2% and 5% every day.

The UK estimates of R and growth rate are averages over different epidemiological situations and should be regarded as a guide to the general trend rather than a description of the epidemic state. Given the increasingly localised approach to managing the epidemic, particularly between nations, UK level estimates are less meaningful than previously.

These estimates represent the transmission of COVID-19 over the past few weeks due to the time delay between someone being infected, having symptoms, and needing healthcare.

Latest by NHS England regions

These are the latest R and growth rate estimates by NHS England regions.

Region R Growth rate % per day
England 0.8 to 1.0 -4 to 0
East of England 0.7 to 1.0 -5 to 0
London 0.7 to 0.9 -5 to -1
Midlands 0.7 to 1.0 -5 to 0
North East and Yorkshire 0.8 to 1.0 -4 to 0
North West 0.7 to 1.0 -4 to -1
South East 0.7 to 1.0 -5 to 0
South West 0.7 to 0.9 -6 to -1

When the numbers of cases or deaths are at low levels and/or there is a high degree of variability in transmission across a region, then care should be taken when interpreting estimates of R and the growth rate. For example, a significant amount of variability across a region due to a local outbreak may mean that a single average value does not accurately reflect the way infections are changing throughout that region.

It is SAGEs expert view, however, that this weeks estimates are reliable.

Estimates for R and growth rates are shown as a range, and the true values are likely to lie within this range.

See a time series of published R and growth rate estimates (ODS, 21.8KB) from 29 May 2020 for:

  • UK
  • England
  • the 7 NHS England regions

The time series document is updated regularly.

Latest for devolved administrations

The latest ranges for R values in the devolved administrations are published on their respective websites:

Publishing estimates over Easter

The government intends to publish R and growth rate estimates on Friday 2 April (Good Friday).

Other key statistics

The ONS Infection Survey provides information on:

  • the number of new infections of the disease identified during a specified time period (incidence)
  • the proportion of the population that test positive for the disease in the community at any given point in time (positivity rate or prevalence)

Other data on testing, cases, healthcare, and deaths is available at the Coronavirus (COVID-19) in the UK dashboard.

About R and growth rate

R

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

An R value of 1 means that on average every person who is infected will infect 1 other person, meaning the total number of infections is stable. If R is 2, on average, each infected person infects 2 more people. If R is 0.5 then on average for each 2 infected people, there will be only 1 new infection. If R is greater than 1 the epidemic is growing, if R is less than 1 the epidemic is shrinking. The higher R is above 1, the more people 1 infected person infects and so the faster the epidemic grows.

R can change over time. For example, it falls when there is a reduction in the number of contacts between people, which reduces transmission. R increases when the numbers of contacts between people rise, leading to a rise in viral transmission.

Growth rate

The growth rate reflects how quickly the numbers of infections are changing day by day. It is an approximation of the percentage change in the number of infections each day. If the growth rate is greater than 0 (+ positive), then the epidemic is growing. If the growth rate is less than 0 (- negative) then the epidemic is shrinking.

The size of the growth rate indicates the speed of change. A growth rate of +5% indicates the epidemic is growing faster than a growth rate of +1%. Likewise, a growth rate of -4% indicates the epidemic is shrinking faster than a growth rate of -1%. Further technical information on growth rate can be found on Plus magazine.

How growth rates are different to R estimates

R alone does not tell us how quickly an epidemic is changing. Different diseases with the same R can generate epidemics that grow at very different speeds. For instance, 2 diseases, both with R=2, could have very different lengths of time for 1 infected individual to infect 2 other people; one disease might take years, while the other might take days.

The growth rate provides us with information on the size and speed of change, whereas the R value only gives us information on the direction of change.

To calculate R, information on the time taken between each generation of infections is needed. That is how long it takes for one set of people in an infected group to infect a new set of people in the next group. This can depend on several different biological, social, and behavioural factors. The growth rate does not depend on the generation time and so requires fewer assumptions to estimate.

Neither one measure, R nor growth rate, is better than the other but each provide in



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