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expert reaction to latest R number and growth rates, published by the government

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 weekly revision of the ranges for the estimate of R and the growth rate of infections, from SAGE and the Government Office for Science (GOS), are out a couple of days earlier than usual this week.  Given all the other bad news about the pandemic that we’ve had in the past few days, it’s not at all surprising that the ranges are higher than they were last week, and indeed they’ve now increased for three weeks in a row.  That hasn’t happened since September, and back in September the total numbers of infections were a lot lower than they are now.  This week’s range for R, for the whole UK, is 1.1 to 1.3.  Last week the range was 1.1 to 1.2, so the increase is only at the higher end and is not large.  The range for the growth rate is wide this week.  It’s +1% to +6% per day, and again the upper end is higher than last week when it was +4%.  All these figures indicate that the number of infections is increasing, and very probably increasing faster than last week.  The range for the growth rate is quite wide – it says that the number of new infections each day is somewhere between 1% and 6% higher than the number the previous day.  That wide range is an indication that, at the level of the whole UK, SAGE has to be relatively uncertain about how fast the epidemic is growing, though it’s pretty certain that it is growing.  A 1% daily growth rate corresponds to a doubling time of about 10 weeks, if the growth continues at that rate.  That’s still growth, but reasonably slow.  However, a 6% rate corresponds to a much shorter doubling time of about 12 days, or not much over a week and a half.  If the growth rate really is somewhere near this top end of the range, that’s quite alarming.  But there hasn’t been this much uncertainty in the growth rate estimates since early October.

“It’s important to remember that these R and growth rate estimates are what are known as lagging indicators of the path of the pandemic.  They are based on a wide range of data, including numbers of hospital admissions, ICU admissions and deaths.  If someone is admitted to hospital, or unfortunately dies, that will typically be some time after they were first infected.  Other data that go into the estimation will reflect recent trends in new infections more closely, but these estimates still can’t yet take into account the very recent changes in restrictions on people mixing (like the introduction of Tier 4 in parts of England, or the changes in the rules in some places for Christmas) or the recent information on the possible transmissibility of the new variant of the virus.

“SAGE also, rightly, point out that the trends in numbers of infections are different in different parts of the UK, so that the national R and growth rate ranges present only a kind of average picture.  For England as a whole, the R range is slightly wider than for the whole UK, 1.1 to 1.4, but the growth rate range is slightly narrower, +2% to +5% per day.  You can’t conclude definitely that the R number for England is higher (or indeed lower) than the UK average, or that the growth rate in England is different from the UK.  That’s because the actual growth rate and R number could be anywhere in those ranges, though somewhere in the middle is more likely than somewhere right at one end, and the width of the ranges is related to the uncertainty in the estimates as well as their exact value.  (Yes, I know this is complicated.)  I haven’t managed to find any really up to date estimates of R for the other three UK countries today.

“The England ranges are also averages across a country where the path of the pandemic is different in different regions and areas, so again the ranges for the whole country don’t necessarily tell you what’s going on where you live.  Ranges for the NHS regions of England are also provided by SAGE.  For the two northern regions (North West, and North East and Yorkshire), the R ranges are both 0.9 to 1.1, so R might be above 1 or below 1.  The growth rate range for the North East and Yorkshire is -2% to +2% a day, with -2% meaning that tomorrow’s number of new infections would be 2% lower than today’s.  So there the number of infections may be shrinking or may be growing – a -2% rate means that the number of infections would halve every five weeks, and a +2% means that the number would double every five weeks, or it could be somewhere between and remain roughly the same over time.  For the North West the growth rate range is 0% (no change over time) to +3% (doubling in about 3 weeks).  The position in London, the South East and the East of England is much gloomier – R ranges that start at 1.2 and go up to 1.4 or 1.5, and growth rate ranges that start at +4% (doubling time two and a half weeks) and go up to 7% or 8% (doubling times 9 or 10 days).  The estimates for the Midlands and the South West are somewhere between the two Northern regions and the three South-Eastern regions.”


Prof Paul Hunter, Professor in Medicine, The Norwich School of Medicine, University of East Anglia, said:

“The most obvious aspect of today’s published estimates of R value is that across the UK and especially in England R is above 1 in the range 1.1 to 1.4.  There is also quite marked regional variation.  Those Regions where we know the new variant has taken hold (East of England, London and the South East) have the fastest growth and higher estimates of R (1.2 to 1.5).  It is likely that R will increase in all other regions over coming days as the new variant becomes predominant there as well.”


Prof Liam Smeeth, Professor of Clinical Epidemiology, and Dean of the Faculty of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, said:

“The latest figures do show greater levels of spread in London and the South East, and at least some of this rise could be attributable to the new variant of the virus.  There is pretty strong evidence it is more infectious, so passes between people more easily, and thus spreads more quickly.  This means that low level social distancing measures will be less effective, and to control the spread tighter measures are needed.  In effect, the natural R number of the new variant will be higher, so to get this below 1 will need greater efforts to control spread.  The policy steps taken – Tier 4 and the Christmas restrictions – are, sadly, an appropriate and much needed response.

“The VERY good news is that there is nothing to suggest the existing vaccines will be any less effective against the new variant.  The priorities remain.  These are firstly everyone uniting in collective action to control the spread and limit the damage done by uncontrolled transmission of the virus.  While painful and difficult for everyone, a short period of tighter restrictions now will prevent the devastating effects of uncontrolled spread.  This will speed up how quickly we can get on top of this virus and allow our social and economic lives to return to some kind of normality.

“Secondly, to roll-out vaccination as quickly as possible.  The MHRA, our world leading regulator, has undertaken a rigorous process to ensure the vaccine is safe, and continues to monitor safety very closely.  I urge everyone who is offered the vaccine to have it: this will  protect them but also protect those around them and indeed the whole of society.  Achieving a high level of uptake of the vaccination provides a route out of this terrible pandemic for all.”


All our previous output on this subject can be seen at this weblink:


Declared interests

Prof Kevin McConway: “I am a Trustee of the SMC and a member of the Advisory Committee, but my quote above is in my capacity as a professional statistician.”

None others received.

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