What is the coronavirus R number and is it rising in the UK?

With models suggesting that R could have risen above 1 in some parts of the UK, we look at what that means and how concerned we should be:

What is R?

R is a measure of the average number of people one infected person will go on to infect. If R is above 1, an outbreak can grow exponentially, but if it is below 1, the outbreak shrinks.

What level is it at?

The latest official estimates – produced by a group of researchers called Spi-M and based on multiple models – put R at between 0.7 and 0.9 for the whole of the UK, and between 0.7 and 1 for England alone.

However, research by Public Health England (PHE) and Cambridge University suggests R is on the rise. R is now at around 1 for the south-west and just over 1 for north-west England.

Researchers at the London School of Hygiene and Tropical Medicine (LSHTM) also say the overall R may be creeping up, but put R in the south-west at 1and 0.8 in the north-west.

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Why might R be increasing? And should we be worried?

There are a number of possibilities. One is that as lockdown measures have been eased, community transmission has risen. But speaking on BBC’s Andrew Marr Show on Sunday, Prof John Edmunds, an LSHTM epidemiologist, suggested another reason.

“The large amount of community transmission that was occurring a couple of months ago has not come to a complete stop, but has reduced enormously,” he said. Instead, he argued, the epidemic has been concentrated in settings such as care homes and hospitals, where infections spread efficiently, meaning they are now influencing the average R to a greater degree, and hence R appears to have crept up.

While spread of the virus in hospitals and care homes is of deep concern, and could seed infections in the community, Edmunds’ explanation suggests the relaxation of lockdown measures is not the main driver of the upward creep in R, and different measures may be required to tackle the trend.

However, Edmunds said the key question that remains is whether transmission in the community is also rising – something he says is unclear, despite the PHE/Cambridge team arguing that the rise in R “is probably due to increasing mobility and mixing between households and in public and workplace settings”.

“They actually have that embedded in their model, that assumption,” said Edmunds, noting that the team drew on Google mobility data, which shows people moving around more. “That doesn’t necessarily mean that contact between individuals has been going up by anywhere near the same amount.

“We have been measuring people’s contact behaviour over the last two months or so and that has crept up a … very small amount, but hasn’t returned to anywhere near pre-lockdown levels.”

When asked at a press briefing on Friday whether he believed R was above 1 in the community anywhere in the UK, Dr Paul Birrell, a co-author of the Cambridge study, said it was hard to say with certainty, “but my strong suspicion is that R is below 1 in the community, but is increasing”.

What about north-west England?

The PHE/Cambridge study reveals that while the median figure for R is 1.01 in the north-west, the full range of possible values is 0.83 to 1.18. Similar ranges are seen for the other regions.That means that it is not certain whether R is truly above or below 1 anywhere.

Edmunds noted that other models give different figures: “I would stress that overall, the assessment is that it is still below 1 probably everywhere on average.”

Related: Easing the lockdown: how will we know if infections bounce back?

What should we be looking out for?

Recent lockdown relaxations are expected to be reflected in R figures over the course of the coming week.

Also important to consider is the number of infections. When they are widespread across the community, an R above 1 is very concerning as it could mean a rise in cases that spreads to new areas – as happened at the start of the outbreak. Infection rates in the north-west remain high, so it is important to establish whether R in the community there is indeed above 1.

However, as the overall number of infections in the community reaches low levels, a large R may arise from a local outbreak or cluster of infections – a very different scenario.

Prof Rowland Kao, an epidemiologist at the University of Edinburgh, said: “If R equals 1.1, and you have 10 infected, that is only 11 new cases, which means that ramping up efforts can easily contain it – if 1,000 are infected, that is 1,100 more, which is much harder.”

If R is sustained above 1, however, the situation could rapidly deteriorate. Kao noted repeated outbreaks could also raise the risk of the infection spreading to new areas or triggering a new outbreak in a hospital or care home.

In addition, experts warn, at low levels of infection, uncertainty in R increases.

Dr Sebastian Funk of the LSHTM said the UK government appeared to be happy with R floating around 1, despite infections remaining relatively high. “That will come with the risk of higher number of deaths and morbidity compared to a scenario where you drive the disease to lower numbers before loosening the physical distancing measures,” he said.