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

Department Of Health

April 2
12:30 2021

Latest R and growth rate

No UK estimates for R and growth rate have been agreed by SAGE this week

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 and may not accurately reflect the current picture of the epidemic.

The R value and growth rates for the 4 nations and NHS England regions are more robust and useful metrics than those for the whole UK.

An R value between 0.8 and 1.0 means that, on average, every 10 people infected will infect between 8 and 10 other people.

A growth rate of between -4% and 0% means that the number of new infections is broadly flat or shrinking by up to 4% every day.

These estimates represent the transmission of COVID-19 over the past few weeks due to the time delay between someone being infected, developing 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 -6 to -1
London* 0.7 to 1.0 -5 to -1
Midlands 0.7 to 1.0 -5 to -1
North East and Yorkshire 0.8 to 1.0 -4 to 0
North West 0.8 to 1.0 -4 to 0
South East 0.7 to 1.0 -5 to -1
South West* 0.7 to 1.0 -5 to 0

* Particular care should be taken when interpreting these estimates, as they are based on low numbers of cases and/or dominated by clustered outbreaks. They should not be treated as robust enough to inform policy decisions alone.

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.

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, 23KB) 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:

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 d

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