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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.


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

“With the ONS data only covering the period up until 2 January 2021, these data are unaffected by closing down of schools and the national lockdown in England introduced earlier in this week.

“The fact that now almost all regions of England and the UK more generally have very similar values of R and the growth rates can probably be explained by the new variant of the virus spreading much more widely and becoming established in all of the regions, which means that their local growth would now proceed at similar rates.

“Since, on average, it takes about 5.5 days from infection to appearance of symptoms and even longer to hospitalisation, possible mixing of people at Christmas and New Year is only now starting to influence the numbers of new cases and hospitalisations.”


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

“R estimates are based on data up to 4 January 2021, and with very limited data collected during the festive period, there is greater variability in the estimates of the R number.

“The fact that R number exceeds 1 in all regions of England is indicative of the new variant of the virus firmly establishing itself around the country and overcoming earlier protection measures, which, because more infections generally means more hospitalisations, is further manifested by a significant increase in hospitalisations all around England over the last couple of weeks.

“Hopefully, the new measures introduced earlier this week will be sufficient to stem this growth and prevent the numbers of hospitalisations from growing too fast to overwhelm the hospital capacity.”


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

“The weekly revisions to the R number and growth rate estimates, from SAGE and the Government Office for Science (GOS), are back to their normal weekly publication pattern after a week off for the holidays.  As always, the estimates are produced in the form of a range of values, not a single number, because there is considerable uncertainty in the figures and a single number would hide that.  This week’s range for R for the UK as a whole is 1.0 to 1.4, meaning that on average, 10 infected people would infect between 10 and 14 other people.  If the true average were 10, R would be 1.0 and the number of infections would be neither growing nor falling, but if R is bigger than 1.0, the number of infections will grow.  The last range for R, published on 23 December, was 1.1 to 1.3.  Today’s range is extended from that at both ends.  That means, roughly, that SAGE don’t feel they have clear evidence on whether R has got bigger or smaller since the last publication, but their uncertainty about the R value has become greater.  As they always have done for many weeks, SAGE point out that in any case the R number for the whole of the UK isn’t a very meaningful measure.  It’s an average over regions and countries which differ quite considerably in the way the epidemic is changing, and which have different administrations with different policies.  Indeed those policies can vary between different parts of the same UK country – that’s not the case in England now, but it was until the start of this week.

“The fact that the lower end of the R interval is at 1.0 does make it more likely, in the opinion of SAGE and its advisers, that the national average R could be as low as 1.0, and it’s even possible (though very unlikely) that it could be slightly lower than 1.0.  But, at the other end, it’s also possible that it could be 1.4 or even slightly higher.  The last time that the upper end of the UK R range was at 1.4 was back on 23 October, when infections were growing at quite a fast rate but had not yet reached anywhere near the level at which they now are.  So the news on R isn’t good.

“The range for the UK-wide growth rate is 0% to +6% a day.  If the rate were exactly 0%, the number of infections on one day would be roughly the same as the day before.  If it were +6%, then there would be 106 infections tomorrow for every 100 infections today, and the number of infections would be growing quite fast.  A +6% growth rate, if it continued at that level, would mean that the number of infections doubled in about a week and a half.  That would be a dangerously high rate of growth, particularly because (other things being equal) that could mean that the number of hospitalisations and the number of deaths would also double every week and a half.  Of course, +6% is the upper end of the range and the growth rate is likely not to be that high – but even a +3% growth rate would mean a doubling of infections every three weeks or so, and that’s not sustainable.  None of these estimates will yet have been able to take into account the very recent lockdowns, and they also won’t have fully taken into account, for example, the introduction of Tier 4 restrictions across most of England shortly before Christmas.  That’s because, to some extent, they are based on data, such as numbers of hospitalisations and deaths, that lag behind changes in numbers of new infections.

“The release also gives ranges for the R number and growth rate for England, and for the different NHS regions in England.  The R number range for England is 1.1 to 1.4, so it’s placed slightly higher than the average R range for the UK.  That may be related to the latest results, also published today, from the ONS Infection Survey, which indicated that infection rates in Wales and, possibly, Northern Ireland, might be decreasing.  That could mean that the rate of increase in England is a bit higher than the UK average.  What’s depressing, though, is that the R ranges for all of the English regions apart from the North West are entirely above 1.0.  (The range for the North West is 1.0 to 1.4, so just does let in the possibility that R there is 1.0 or slightly less.)  Also, the lower end of the growth rate ranges for England as a whole, and for all the English regions except the North West, are above 0.  Again, these ranges can’t fully take into account recent changes on lockdowns and other rules, but they do give yet another reason why action to stop the growth of the epidemic is necessary.”


Dr Simon Clarke, Associate Professor in Cellular Microbiology at the University of Reading, said:

“The fact that the national R number was over 1 for the period in question is completely unsurprising, given as we already know that the real-life infection numbers were increasing at pace.  It should be remembered this R number is an estimate, diagnoses of infections are real.  The lack of substantial variation amongst the English regions is indicative that the epidemic was thought to be growing right across the country and that the measures required to suppress growth were probably needed everywhere.”



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.”

None others received.

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