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Source: United Kingdom – Executive Government & Departments

The government have published their latest estimates for the COVID-19 growth rate and R value.

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

“The latest estimates of the UK’s R number and growth rate, from SAGE and the Government Office for Science (GOS), have been published.  As always they don’t give a single figure, but instead give ranges within which they think it’s likely that each of the numbers will fall.

“This week’s range for R is from 1.2 to 1.5, meaning that (on average) 10 infected people in the UK will infected between 12 and 15 others.  That obviously means that infections would increase, which isn’t good news.  However, the range is a little lower than last week, when it was 1.3 to 1.6.  While this looks like good news – albeit not particularly good because the range hasn’t changed much and R is still above 1 – we’ve got to be very careful in interpreting it.  It does not mean that R has definitely gone down.  Indeed the ranges include the possibility that R might even have gone up – it could have been 1.3 last week and 1.4 this week, for example.  But at least it allows the possibility of a move in the right direction.

“This week’s range for the growth rate in infections is +4% to +9% per day.  That’s very little changed from last week, when it was +5% to +9% per day, so it allows the possibility that the growth rate might have fallen slightly, but we can’t be sure.  A 4% growth rate would mean that the number of cases doubles in about 18 days, while 9% would mean a doubling in 8 days.  Even a doubling in 18 days is quite a fast rate of increase, it has to be said.

“Today’s preprint from the REACT-1 study also produced estimates of R and the growth rate, based on their swab tests on a representative sample of the English population taken between 18 September and 5 October.  They gave central estimates for R and the growth rate, but probably the most appropriate thing to compare with the SAGE/GOS estimates is the REACT-1 ‘credible intervals’.  These give ranges for R and the growth rate that are consistent with their data, allowing for the fact that it’s a sample and not everyone in the country was tested, so there’s some statistical uncertainty.  The REACT-1 range for R runs from 1.05 to 1.27, so that’s somewhat lower that this week’s SAGE/GOS range for England (1.2 to 1.5, as for the whole UK) – though the ranges do overlap so the two sets of estimates are not actually inconsistent with one another.  The REACT-1 range for the daily growth rate runs from +0.8% to +4.0%, again somewhat lower than SAGE/GOS (a range of +4% to +8% for England), and indeed not quite overlapping.  On doubling time, REACT-1 gives a range from 17 days to 84 days – very wide, but again indicating rather slower growth than SAGE/GOS.

“One important possible reason for the differences is that the estimates from SAGE and GOS use a much wider set of sources of data that REACT-1, which bases its estimates entirely on the rate at which the daily numbers of positive swab tests was growing during the latest round of its survey.  The SAGE/GOS estimates also use data from other sources, for example on hospital admissions and deaths.  This allows more sophisticated models to be used.  However, it also means that some of the data lag behind the current position – for instance, it would take some time after a person is infected before they might have to go to hospital, and longer till their death, if, sadly, that happens.  Therefore the SAGE/GOS estimates may not yet reflect recent changes that affect the transmission rate.  Since the REACT-1 estimates are based on data going right up to only 4 days ago, it’s possible that they are more up to date with the current position – but they may be less reliable because they are based on only one source of data.  It’s good that we are getting estimates from different sources, though it does emphasise the inescapable uncertainties involved.”

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.”

MIL OSI United Kingdom