The government have published the latest estimates for the COVID-19 R number and growth rates.
Prof Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:
“SAGE have revised their ranges for the R number and the growth rate of infections in England and in the English NHS regions. They no longer produce estimates for the whole UK, and indeed this week they say that we should “take particular care” in interpreting the estimates for six of the seven NHS regions, because they are based on low numbers of cases or deaths and/or are dominated by clustered outbreaks. So basically SAGE are saying that these figures shouldn’t be used as a basis for policy decisions, and I’d argue that they don’t really describe anything very useful. They are averages for a whole country (England), or very large parts of a country (the regions), that probably average out figures that vary a lot from one place to another, and can’t tell you much about what’s happening where you live or work. Also, vaccination means that the relationship between new cases and serious illness or death is changing radically, so that the concerns that we rightly used to have about the R number are much diminished.
“That said, what’s changed compared to last week? The R range for the whole of England is 0.7 to 1.0. Last week, and for the two weeks before that, it was 0.8 to 1.0. So SAGE are saying that they are less certain than they were last week about R, because the range is wider, but they are still pretty confident that it’s not greater than 1.0, so that the number of cases isn’t increasing. We can’t be sure that R has gone down – for example, if it was 0.9 both weeks, that’s within both ranges – but it might well have gone down a bit. The growth rate range for the whole of England this week is -6% to -1% per day. A growth rate of -6% per day means that there would be 6% fewer cases tomorrow than today, so for every 100 new infections today, there would be about 94 tomorrow. If that rate of decrease continued over time, new infections would halve in roughly a week and a half. At the other end of the range, a growth rate of -1% would mean 99 cases tomorrow for every 100 today, and at that rate, cases would halve in about 10 weeks. Last week’s range for the growth rate for England was -4% to 0% per day. This week’s range is wider, so there’s more uncertainty, but both of the ends are lower. That makes it likely (though not certain) that new cases are decreasing a bit faster than last week. Also, the upper end of last week’s range, 0%, indicated that SAGE thought it just about possible that infections were not decreasing – this week the upper end is below zero so that shows much greater confidence that cases are really going down.
“Given the serious provisos that SAGE have given about the meaning of the regional ranges, it’s arguably hardly worthwhile discussing them, so I’ll be brief. Most of the R ranges and growth rate ranges for regions have changed slightly compared to last week, and what’s slightly concerning is that the upper ends of two of them, for London and for the South West, are now 1.1, so above the threshold of 1.0. This doesn’t concern me hugely though. That’s partly because SAGE have said they don’t clearly represent the detailed position in those regions anyway, partly because the growth rate ranges for both those regions don’t go above the 0% figure indicating no change either up or down, and partly because, according to the ONS infection survey results that came out earlier today, the South West region has the lowest test positivity rate in England, so that a small percentage increase in infections there would not affect many people. (The positivity rate in London is rather higher, according to ONS, but still is below the England average.) SAGE’s growth rate ranges for all the other NHS regions have an upper end that’s below zero, indicating that SAGE is pretty confident that infections are decreasing there, on average. But given SAGE’s proviso about possible outbreaks, that certainly doesn’t mean that infections are falling in every part of those regions.”
All our previous output on this subject can be seen at this weblink:
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.”