The government have released the latest estimates for the COVID-19 R value and growth rates.
Prof Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:
“The latest revisions to the ranges for the R number and growth rates are, on the whole, pretty good news. They certainly indicate a continuing pattern of decreasing infections across the UK. The range for R for the whole UK is slightly different from last week. This week it’s 0.7 to 0.9, while last week it was 0.6 to 0.9. Both ends of the range are below 1.0, indicating that it’s very likely that infections are decreasing. There’s a temptation to think that things are looking worse because one end of the range has increased. But that’s certainly not the position, not necessarily anyway. The range is a bit narrower this week than last, which indicates that SAGE (who produce these figures) are rather more certain about the R number than they were last week. And the R number itself isn’t a range – SAGE give a range for it to illustrate that it can’t be known exactly. Maybe it hasn’t changed at all since last week – for example, if the actual R number were 0.8 both last week and this, that would be inside both ranges and compatible with what SAGE are saying. It’s also important to remember that R doesn’t tell us how many people are getting infected. It’s more like a measure on a speedometer, in a sense. When it’s below 1, it tells us (in a slightly complicated way) how fast the number of infected people is falling. R doesn’t have to get smaller for the number of infected people to continue to decrease, any more than the reading on your speedometer has to change for you to continue to go forwards.
“The growth rate figures perhaps illustrate this more clearly. This week’s range for the growth rate for the whole of the UK is -5% to -3%. That means that SAGE estimate that the number of new infections each day will be somewhere between 5% and 3% smaller than the number of new infections the day before. If the number is falling every day, that means that there are fewer and fewer infections as time goes on – and that would be true even if (say) the rate stayed exactly at -4% (4% decrease each day) for a long time. Last week’s growth rate range for the UK was -6% to -2% per day. This week’s range is centred in exactly the same place, but it’s narrower, with the top end being lower and the bottom end being higher. This means that SAGE aren’t really indicating any overall change in how fast the numbers of infections are falling, but they are more certain about what that speed of decrease is. If the growth rate were -5% (5% daily decrease) for a long time, the number of new infections would halve roughly every two weeks. A -3% growth rate would mean the numbers halved roughly every three weeks. These are reasonably fast rates of decrease.
“This week’s R range for England as a whole is 0.7 to 0.9, the same as last week, and indeed the range hasn’t changed since 5 February. Both ends are comfortably below 1. The R ranges for all but one of the English regions are also entirely below 1, and indeed most of them have not changed since last week. Both ends of the range for the South East have got smaller – it’s 0.6 to 0.8 today, and was 0.7 to 0.9 last week. The range for the North East and Yorkshire is 0.7 to 1.0 this week – last week it was 0.7 to 0.9. So, for that region, SAGE feel that there’s a possibility that R might be at the level where infections could start to grow. But most of the range is below 1, indicating that infection numbers are probably falling there too. The growth rate ranges for England as a whole and for all the English regions are entirely in the negative numbers, corresponding to numbers of new infections decreasing from day to day.
Further information
“As always, this data release does not include separate figures for R in the UK countries outside England, though it does give links to the relevant estimates from the devolved administrations in the other countries. For all three countries, the current R ranges are entirely below 1.
“The Imperial College REACT-1 survey, which published a new report1 this week, also provides estimates of the R number and the daily growth rate. Just based on the decrease in infections between the two parts of round 9 of REACT-1 (which took place between 4 and 23 February), the researchers estimated that R is between 0.76 and 0.97, which is not too different from the Government’s current range for R for England (0.7 to 0.9) – but the REACT-1 researchers are confident that, nationally, R was below 1. The estimated growth rate from those two part-rounds of REACT-1 is between -4.1% and -0.4% per day, which is again not too different from the Government’s current range for England of -5% to -3%, and still clearly indicates decreasing infections (because of the minus signs). Overall, the estimates from REACT-1 were slightly more pessimistic than the latest Government estimates, but they are still clearly indicating that infections are decreasing nationally, on average. Also, the SAGE estimates for R and the growth rate are based on a considerably wider set of sources of data than the REACT-1 estimates, which are based solely on the percentages of positive swab tests in the people they test in their survey.”
1 https://spiral.imperial.ac.uk/handle/10044/1/86343
Prof John Edmunds, Professor in the Centre for the Mathematical Modelling of Infectious Diseases, London School of Hygiene & Tropical Medicine, said:
“I think that the era of the R number is coming to an end. At various points in the previous year Government policy has been aimed at trying to reduce R and maintain it at or below one. This is no longer the case. The recently published Roadmap has this as one of its four tests: “Infection rates do not risk a surge in hospitalisations which would put unsustainable pressure on the NHS”. That is, an increase in infections may be tolerated as long as it doesn’t put undue pressure on hospital services. The idea is that even if infection rates increase the numbers of new hospitalisations will hopefully remain low as the highest risk groups have been vaccinated. The growth or decline in hospitalisations is now critical, not the overall R number.”
https://www.gov.uk/guidance/the-r-number-in-the-uk
All our previous output on this subject can be seen at this weblink:
www.sciencemediacentre.org/tag/covid-19
Declared interests
Prof Kevin McConway: “I am a Trustee of the SMC and a member of its Advisory Committee. I am also a member of the Public Data Advisory Group, which provides expert advice to the Cabinet Office on aspects of public understanding of data during the pandemic. My quote above is in my capacity as an independent professional statistician.”
None others received.