Sage 'not confident' R number below 1 in England

Government advisors are "not confident" the R number is below one in England after early estimates were published today.

The figures estimate the growth rate of coronavirus transmission, which reflects how quickly the number of infections is changing day by day, may be increasing.

The stats were published on Friday by the Government Office for Science and the Scientific Advisory Group for Emergencies (Sage) has shown the growth rate to be between minus 4% to minus 1, a slight change from between minus 5% to zero last week.

The R number represents the number of people each Covid-19 positive person goes on to infect.

In England, the R is offically between 0.8 and 1, but Sage has indicated it is not confident that R is currently below one in the region.

Dr Yuliya Kyrychko, reader in mathematics at University of Sussex, said: "What this means in practical terms is that the situation is pretty much on the precipice, and then it can develop very rapidly."

She added daily infections had been steadily increasing since the middle of July, meaning "that it is very likely that the view of Sage on R number not being below 1 is correct."

The estimates for R and growth rate are provided by the Scientific Pandemic Influenza Group on Modelling (SPI-M), a subgroup of Sage.

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It said: “We are starting to see early indications that these values may be increasing."

“This is not yet reflected in these estimates because the data used to calculate R and growth rate reflect the situation from a few weeks ago.”

The delay between infection and the need for hospital care means it takes two to three weeks for the changes in the spread of Covid-19 to be reflected in the estimates.

But models that use Covid-19 testing data, which have less of a time delay, indicate higher values for R in England, the Government Office for Science statement said.

It added: “For this reason, Sage does not have confidence that R is currently below one in England.

“We would expect to see this change in transmission reflected in the R and growth rate published over the next few weeks as we gain more certainty of what is currently happening.”

However, the Government officials and advisers said is also important to recognise that these are estimates, and there is a high degree of uncertainty with them.

They said these figures are less useful in determining the state of the epidemic when disease incidence is low and local outbreaks could be contributing to the variability in the numbers.

Commenting on the figures, Dr Daniel Lawson, lecturer in statistical science at University of Bristol’s School of Mathematics, said: “Sage appropriately warn they are not confident that R is currently below 1, for two reasons.

“Firstly, there is significant uncertainty in the data, and modelling this is very difficult.

“Secondly, the data are observed with a time lag, so we can only know R in the past with certainty.

“R is very close to 1, and these uncertainties combine to make it plausible – though not yet likely – that R is currently above 1.”

Meanwhile, the reproduction number, referred to as R, for the UK as a whole remains unchanged at between 0.8 to 1.

Across England, the growth rate is between minus 4% and zero, compared with between minus 3% and zero last week.

Regional R values also appear to be close to 1.

In the North West, where most of the area is under regional lockdown, the R number is between 0.8-1.1 with the growth rate unchanged between minus 3% to plus 1%.

The rest of England is a mixed picture:

• In the East, the R number is 0.8-1 and the growth rate has changed from between minus 4% to minus 1% last week, to between minus 5% to minus 1%.

• In London, the number is 0.8-1 and the growth rate has changed from between minus 4% to minus 1% last week, to between minus 5% to minus 1%.

• In the Midlands, the R number is 0.8-1 and the growth rate has changed from between minus 5% to minus 1%, compared with between minus 3% to zero.

• In the North East and Yorkshire, the R number is 0.8-1 and the growth rate remains unchanged since last week between minus 4% to zero.

• In the South East, the R number is 0.8-0.9 and the growth rate is between minus 5% to minus 1%, compared with minus 4% to zero.

• In the South West, the R is 0.8-1 and its growth rate has declined slightly from between minus 3% and plus 3% last week to between minus 5% and plus 1%.