The south west of England has the highest rate of coronavirus spread in the UK, with an “R number” estimated to be in the range of 0.8 to 1.1. Most other regions in England have R numbers whose range goes up to 1, according to government figures released today that provide regional R values for the first time.
The R or R0, which stands for the effective reproduction value for a disease, reflects the rate at which the infection is spreading through a population. When coronavirus first reached the UK, on average one infected person passed it to about three others, giving it an R of 3. With an R of 1, case numbers would be steady. If R can be pushed down to less than 1, through social distancing and self-isolation measures for instance, the disease should eventually peter out.
There has been intense interest in tracking countries’ R numbers as a way of assessing their progress against coronavirus. However there is a time lag, as it can take two or three weeks for people to realise they’re infected and get tested. This means that the newest R numbers are usually a reflection of the situtation several weeks ago.
Another problem is that R is less accurate when it is based on smaller case numbers, and new cases have been falling in the UK since early May. Breaking the numbers down by region also makes R less accurate, the government’s science advisers have said.
“When there are a small number of cases, estimates of R become less useful in determining the state of the epidemic. R is impossible to estimate with accuracy and will have wide confidence intervals that are likely to include 1,” the government said in a statement.
In the next few weeks, the government will start releasing regional new infections and prevalence figures, which advisers say are more informative.
In China, where the coronavirus outbreak began, numbers of new cases are still falling, but much more slowly than they were. “If you look at many epidemic curves, it’s very common to see a long flat tail,” says Mark Woolhouse, at the University of Edinburgh.
In that phase, R would be just less than 1 for a long time. And when small clusters happen, such as within nursing homes, this could temporarily push R up to more than 1 locally. “We are approaching a time where R is not that useful,” says Woolhouse.
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