September 18, 2020

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

The government have released the latest estimates of the COVID-19 R value.

Dr Yuliya Kyrychko, Reader in Mathematics, University of Sussex, said:

“An increase in R number considered alongside the ONS data survey published today shows that the community incidence went from 0.08% to 0.13% in two weeks from 27th August to 10th September, so the percentage of people testing positive doubled in that time, and this cannot be attributed to higher levels of testing.

“More worryingly, in the same two week time period, the ONS report estimates that infections doubled in almost all age groups apart from 70+ and 12-16 year olds, so the trend is shifting from mainly young people testing positive that we saw recently.

“This suggests that it’s only a matter of time before the infections start spreading to the most vulnerable populations, which will inevitably lead to higher death rates and potentially could overwhelm the NHS.”

Dr Konstantin Blyuss, Reader in Mathematics, University of Sussex, said:

“The latest data on R number in different parts of England and the whole of the UK suggest that the epidemic is no longer localised, but rather the prevalence is growing in all of the regions.

“Combined with an observation from SAGE that the prevalence is doubling around every seven days, this means that the overall epidemic numbers are growing very significantly.

“The fact that the R number is above one in all regions of England suggests that it is very unlikely that the epidemic could now be stopped by introducing local measures, simply because they would not be effective at stopping epidemic growth in other regions.

“The comment from SAGE about death rates being low are consistent with data observed in other countries that are ahead of the UK in terms of the stage of the epidemic, with majority or cases recently being among the younger population.

“But the latest data from the ONS suggest that this trend is changing, and the rates of infection among older age groups have increased significantly over the last few weeks.  Thus it is vital to monitor the rates of disease spread to avoid the potential growth of hospitalisations and deaths.”

Prof Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:

“SAGE and the Government Office for Science (GOS) have published their weekly ranges for the R number and for the growth rate of the COVID-19.  Unsurprisingly, given all that’s been said and published about the current state of the pandemic, the ranges are higher than they were last week.  The UK R number range is 1.1 to 1.4, compared to 1.0 to 1.2 last week, so there’s very, very little doubt that it is now above 1 and therefore that the epidemic is increasing.  The growth rate range runs from +2% to +7%, indicating that GOS and SAGE believe that the number of new cases is growing by between  2% and 7% each day.  (Last week’s range was from -1% to 3%, allowing some possibility that the number of new cases might still be falling slowly.)

“This is undoubtedly concerning, and particularly so when we take into account that the data behind these estimates come from several sources, many of which (such as hospital admissions, admissions to intensive care, and deaths) lag behind the growth in new infections, because it takes time for people to become ill enough to require hospital treatment or, sadly, to die.  So the estimates cannot take into account very recent changes in the patterns of new infections.

“A daily growth rate of 2% corresponds to a weekly growth rate of about 15% (a bit higher than 7 times 2 because each day’s increase is a percentage of the previous day’s already increased figure – a bit like compound interest).  A daily growth rate of 7% corresponds to a weekly growth rate of about 60%.  So the range given for the growth rate is not as high as the doubling of new cases in a week that has been mentioned in the media.  We must hope that recent interventions and policy changes will mean that it does not get up to that level in future.  Even at growth rates within the estimated range, the number of new cases could grow to high levels quickly if the interventions are not sufficiently effective.

“The R number and growth rate ranges for England alone are very similar to those for the whole UK; 1.2 to 1.4 for the R number and +3% to +7% for the daily growth rate.  Estimates for the English regions indicate that the R number is above 1 for all of them, just possibly except the South West.  There, case numbers are low, so that estimates cannot be very precise and the regional R range runs from 0.9 right up to 1.6.  The regional growth rate estimates suggest that numbers of infections are very likely growing in every region of England, though it’s just about possible on these estimates that numbers are stable in the South West and the East regions.  The R number estimates published by the devolved administrations also show a (quite slowly) growing epidemic in Northern Ireland and in Scotland.  In Wales the R range from SAGE was lower, 0.7 to 1.0, but the group specifically advising the Welsh government reported last week¹ that they believe the true R number there is higher.”

1 https://gov.wales/sites/default/files/publications/2020-09/advice-summary-11-september-2020.pdf

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 member of the SMC Advisory Committee, but my quote above is in my capacity as a professional statistician.”