February 26, 2021

expert reaction to latest R number and growth rates estimates, 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:

“Generally, the latest Government revisions to their estimates of the R number and the growth rate of new infections are reasonably good news, in my view.  The range for R for the whole UK is unchanged – it’s 0.6 to 0.9, so still entirely below the threshold number of 1, so that SAGE consider that the epidemic is very likely to be decreasing.  That’s in line with the ONS infection survey results, and with the number of new confirmed cases on the dashboard at coronavirus.data.gov.uk.  (There has been some concern recently that perhaps the numbers of new cases on the dashboard are not decreasing as fast as they were, but they are still decreasing.)

“There is a small revision to the range for the growth rate for the whole UK.  This week it is -6% to -2%, so that SAGE estimate that the number of new infections each day is somewhere between 6% and 2% lower than it was the previous day.  Last week’s range was -6% to -3%, so that the upper end of the range is slightly higher this week.  That doesn’t necessarily indicate that new infections must definitely be falling more slowly – the ranges are compatible with the true rate of decrease being 4% a day in both cases, for example.  It does, however, indicate that SAGE are slightly more uncertain about the rate of change than they were last week.  But probably the most important thing is that both ends of the growth rate range are clearly negative, and a negative growth rate means that new infections are falling.  A growth rate of -6%, if it continued at that level, would mean that new infections would halve in about a week and a half, so pretty rapidly.  A growth rate of -2% would correspond to infections halving in about five weeks.  So there’s no indication at all here of increasing infections, but some caution is certainly indicated.

“This week’s R ranges for England on its own, and for each of the English regions, are also entirely below 1.  That position is arguably slightly better than last week, when the range for one region went right up to 1 (though not above 1).  The growth rate ranges for England, and for the English regions, are also all entirely below zero, indicating that new infections are estimated to be falling in all of them.  There have been some small changes in the R and growth rate ranges for individual regions, though nothing very marked, and broadly speaking the position on all these estimates is much the same as last week.

Further information

“This data release, as always, does not directly cover the position in the UK countries other than England, though it provides links to the relevant information from the devolved administrations.  For Wales and for Scotland, the ranges for R are entirely below 1 (as for England and for the UK as a whole).  For Northern Ireland, the position is not so clear.  The Northern Ireland government does not normally give a single range for R, but instead produces separate ranges estimated from different sources of data, and some of those do go right up to 1 or slightly above 1.  This therefore includes the possibility that numbers of infections are growing there, and that’s a change from the position shown by the previous week’s ranges.  However, the main body of all the ranges is still below 1, indicating that infections are probably still decreasing in Northern Ireland.  In very recent days, the numbers of new confirmed cases in Northern Ireland on coronavirus.data.gov.uk appear to have plateaued – not increasing, but not clearly decreasing either.  But they were definitely decreasing before that, and the ONS Infection Survey bulletin published today showed a very considerable decrease in estimated positivity in Northern Ireland in their most recent week (ending 19 February) compared to the week before that.  So the position in Northern Ireland isn’t so clear and invites some caution.  There are two features of Northern Ireland that can make trends rather more difficult to determine than in the rest of the UK.  First, the population is relatively small, so that there is inevitably more statistical variation from day to day and week to week than in the larger UK countries.  Second, the long land border with the Republic means that potential cross-border movement of infection could be relevant – but I do not have specific expertise on that aspect myself.”

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.”