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 at The Open University, said:

“The Government’s latest revisions of the ranges for R and the growth rate of infections are good news, but not that good. The range for R for the whole UK is 0.7 to 1.0, so the upper end (which was 1.1 last week) has come down slightly, and the whole range is now below 1.0. That means that it’s very likely that the true value of R for the whole country is below 1. If that continues, the pandemic will shrink rather than grow. Why is isn’t entirely good news is that it’s possible that the R is very close to 1. If that’s the position, the number of infections would shrink very slowly. These numbers do not tell us how many infections are actually out there. But we know from the ONS infection survey results, published earlier today, that there are still a lot of infected people. ONS estimate that, for the week ending 30 January, about 1 in every 65 people in England and in Northern Ireland would test positive for the virus, about 1 in 70 in Wales, and about 1 in 115 in Scotland. Those are still very high rates of infection (though less so in Scotland). If R is indeed very close to 1, then it could take a long time to get down from these dangerously high infections rates to something more manageable. Vaccination will help, but we’ve still got a long way to go.

“The growth rate figures are slightly more encouraging. The range for the UK is -5% to -2% a day. A negative growth rate is actually a rate of decrease. So a growth rate of -2% means that the number of new infections tomorrow will be 2% less than the number today, that is, 98% of the number today. If the decline continued at that rate for some time, the daily number of new infections would halve in around 5 weeks. If the growth rate is -5%, and that continued, the daily number of new infections would halve in around 2 weeks. That would be good, but that’s the most favourable end of the range and we can’t be at all sure than infections will fall that fast – though I very much hope they do. And at least both ends of the growth rate range do correspond to decreasing numbers of infections.

“Just looking at England on its own, the R range is 0.7 to 0.9, so the Government is somewhat more confident that cases are falling in England than in the UK as a whole. The growth rate range for England is the same as for the whole UK, -5% to -2% per day. The R ranges for the individual regions of England are mostly completely below 1.0, though 1.0 is the upper limit for two of them, North West, and North East and Yorkshire. And the ranges for the growth rate for all but one of the regions are entirely below 0, indicating that the Government is pretty sure that infections are decreasing. The exception is again North East and Yorkshire, but even there the upper limit of the range is 0 (which means that infections would be neither rising nor falling), so it’s pretty likely that infections are falling there as well, according to these estimates.

“As usual, the regional estimates don’t all tie up perfectly with the results for the English regions from the ONS Infection Survey. That’s because the basis for the estimation is quite a bit different, different data are used, and the timescales are a bit different. (The ONS results relate to a particular week, the week from 24 to 30 January for this week’s bulletin. The R and growth rate estimates take into account data with different timescales, including data on hospital admissions, ICU admissions and deaths. Those measures lag behind numbers of new infections, because it takes time after they are infected for people to get ill enough to need hospital or, sadly, to die. So in a sense the R and growth rate ranges are kind of averages over a period of maybe 3 weeks.) It’s also the case, annoyingly, that the regions that are used by ONS and in these R figures are not quite the same as one another. The ONS survey today did estimate that infections might be rising in the East of England, albeit not very fast, while the R and growth rate figures estimate that they are falling in that region, and the number of new confirmed cases in the East of England on the dashboard at is also falling. We’ll have to see, in future weeks, which trend turns out to be correct.”



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Declared interests

Prof 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.”