February 19, 2021

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

“These revisions of the ranges for R and the growth rate are more good news, on top of the good news from the ONS infection survey earlier today, and the REACT-1 survey earlier in the week. Things are definitely moving in the right direction, and fairly fast. But there is still a long way to go. Vaccinations will help more and more, but things are nowhere near back to normal yet and won’t be for a long time.

“The range for R for the whole UK is now 0.6 to 0.9, and last week it was 0.7 to 0.9. The fall in the lower end of the range doesn’t mean that R has definitely fallen, though it probably has, and this week’s range is wider, indicating that there’s more uncertainty about the exact R number. But SAGE and the Government are still showing confidence that R is below 1. If R stays below 1, the number of cases will not increase. As last week, the range for England is entirely below 1 – it’s 0.7 to 0.9, same as last week – and the ranges for all but one of the English regions are entirely below 1 as well. The exception is the North East and Yorkshire region, where the range is 0.7 to 1.0. That indicates that R for that region is very probably below 1, but there’s a chance it might not be. The latest R estimates for the other three UK countries are also all below 1.

“I tend to prefer looking at the growth rates rather than the R numbers, because they are easier to interpret. It isn’t just a question of whether infections are falling (R below 1) or not – the growth rates give you some idea of how fast the numbers are changing. This week’s range for the growth rate for the UK is -6% to -3%. A negative growth rate means that new infections are falling, and this range indicates that each day’s number of new infections is between 3% and 6% lower than the day before. A daily growth rate of -3%, if it continues at that level, would mean that new infections would halve in around 3 weeks, and a -6% daily rate would mean that new infections would halve in about 11 days. That’s quite a fast rate of decrease, and this week’s range of -6% to -3% is below last week’s range of -5% to -2% at both ends. This indicates that, not only are new infections continuing to fall, they are probably falling faster than they were. The growth rate ranges for the English regions are all entirely in the negative numbers, indicating falling new infections, even for the North East and Yorkshire where the R range goes right up to 1. Pretty good news all round, I’d say.”

Further information

The REACT-1 preprint from earlier this week also produced an R estimate and range, for England (and indeed for the English regions, but a different set of regions than used in these Government ranges so I won’t try to compare them). In fact the REACT-1 researchers produced two sets of ranges, depending on which set of previous results their latest results were being compared with. For the most recent comparison, the REACT-1 range is from 0.69 to 0.76, which isn’t a lot different from the Government’s range for England of 0.7 to 0.9. REACT-1 also estimated a range for the growth rate – it was -5% to -4% per day, again not too different from the Government’s range for England of -6% to -3%. The REACT-1 estimate is entirely backward-looking, comparing their positivity estimate in the most recent period (the first part of February) with their numbers from mid-January, and it’s based only on their data on swabs testing positive. But the Government R and growth rate estimates are also backward-looking to a certain extent. They are based on a much wider range of data, including data on hospital admissions and deaths. Those figures reflect infections a few weeks before the hospital admissions or deaths actually occurred, because it takes a time between someone being newly infected and being admitted to hospital or, sadly, dying, if that happens. Therefore the Government R and growth rate estimates don’t relate to the day they come out, but instead are a kind of average measure over the past few weeks.

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.”