The latest figures for the R value for Covid-19, and growth rates, have been released by the government. R is the average number of people one person infects: if it rises above 1, infections could increase exponentially. Growth rates reflect the change in the number of infections each day; a growth rate of below 0
The latest figures for the R value for Covid-19, and growth rates, have been released by the government.
R is the average number of people one person infects: if it rises above 1, infections could increase exponentially. Growth rates reflect the change in the number of infections each day; a growth rate of below 0 suggests the prevalence of the disease is shrinking.
Once again R for the UK is just below 1, at 0.7 to 0.9. For England alone the range is slightly different at 0.8 at 0.9.
Within England there remains regional variation in R, with London the only region where, according to the statistics, R might have edged above 1 – the range for the capital is 0.8 to 1.1.
The figures reveal that, for the UK as a whole, the growth rate per day is -6% to 0%, with the range at -5% to -2% for England alone.
But once again there are regional variations, with London and the south-west both showing ranges that span 0, at -4% to +2%, and -7% to +2% respectively.
However, the number of infections also matter. If infections are widespread, a rise in R could see cases boom. But if infection levels are low, a large R may reflect a local outbreak or cluster of infections that can be more easily controlled.
Another issue is that as infections fall, it becomes harder to estimate R, meaning the range of values can become wider – a situation also seen if R is calculated for a small geographical area. Similar limitations apply to growth rates.
Prof Rowland Kao, an epidemiologist at the University of Edinburgh, said:
I think the key thing here is that, first of all, the absolute [case] numbers are low enough that even a smallish outbreak will have a direct influence on the reproduction number, R, to send it upwards without it necessarily seeing this being due to an established shift in the epidemic pattern. Also, if these occur in isolated settings, then they will have little impact on generalised transmission in communities.
Kao added that R would not be expected to decline unless new measures were put in place, the virus changed, or immunity was prevalent and rapidly growing.
We are, of course, only starting to see the impact of a combination of relaxation of measures, and changing behaviours from two to three weeks ago.
We’ll have to wait to see if the further relaxations of restrictions will result in enough extra transmission to cause R to rise above 1 consistently and if so, whether our test and trace systems are well prepared to handle this.