The reproduction number, or R value, for Covid-19 is now officially averaging above one across the UK once again – although it varies by region.
The latest R estimate for the whole of the UK is between 1.1 and 1.4. But what does that mean?
What is R?
The reproduction number is a way of rating coronavirus or any disease’s ability to spread.
It’s the number of people that one infected person will pass on a virus to, on average.
Measles has one of the highest numbers with an R number of 15 in populations without immunity.
That means, on average, one person will spread measles to 15 others.
Coronavirus – known officially as Sars-CoV-2 – has a reproduction number of about three if we take no action to stop it spreading.
How is R calculated?
You can’t capture the moment people are infected. Instead, scientists work backwards.
Data – such as the number of people dying, admitted to hospital or testing positive for the virus over time – is used to estimate how easily the virus is spreading.
Why is a number above one dangerous?
If the reproduction number is higher than one, then the number of cases increases exponentially – it snowballs like debt on an unpaid credit card.
But if the number is lower the disease will eventually stop spreading, as not enough new people are being infected to sustain the outbreak.
Governments everywhere want to force the reproduction number down from about three (the R number if we took no action) to below one.
What is the R number in the UK?
The reproduction number is not fixed. Instead, it changes as our behaviour changes or as immunity develops.
Mathematical modellers at Imperial College London attempted to track how the number changed as isolation, social distancing and the full lockdown were introduced in the spring.
Before any measures came in, the number was well above one and the conditions were ripe for a large outbreak. Successive restrictions brought it down, but it was not until full lockdown that it was driven below one.
On Friday 18 September, the government said the R number across the UK was estimated to be between 1.1 and 1.4, slightly up on the previous week.
Are there regional differences?
Scotland’s estimated R number is between 1.1 and 1.4.
In Wales the number is thought to be between 0.7 and 1.2 – while in Northern Ireland it is about 1.2.
In England, the highest estimated R rate is in the South West, where it is between 0.9 and 1.6 – and also in the Midlands and North West, where the rates are between 1.2 and 1.5.
Read More: Coronavirus: What is the R number and how is it calculated?