The R value has sunk below one for the first time since lockdown was introduced, official figures reveal, in the first formal signal that the new restrictions are suppressing the virus.
But how is it calculated? Is it reliable? And how does it vary across regions of the UK?
What is the R rate?
The reproduction, or R rate, is the number of people that a single infected person will go on to infect.
Anything above one, means that each person will be passing the virus on to more than one person allowing the epidemic to grow exponentially.
For example an R rate of 1.5 would mean every 10 people who became infected, would infect 15 more people. Those 15, would, in turn, infect 22.5 people who would then pass the disease on to 33.7.
The opposite is true if the R rate is below one. The Government announced on January 22 that it is somewhere between 0.8 and 1, representing a growth rate of minus four per cent to minus one per cent.
The value was estimated to be between 1.2 and 1.3 on January 15.
Scientists cannot be completely accurate because different models are used to come up with the figures and they do not often agree.
An R number between 0.8 and 1 means that, on average, every 10 people infected will infect between 8 and 10 other people.
If it is at the lower end of the estimate, 0.8, then every 10 people will infect eight others.
If the R rate is at the higher end of one, then the disease has plateaued, so 10 people would infect 10 others, and the epidemic would neither increase nor decline.
A growth rate of between -4% and -1% means that the number of new infections is shrinking by between 1% and 4% every day.
The R rate is calculated by The Scientific Pandemic Influenza Group on Modelling (SPI-M), a sub-group of Scientific Advisory Group for Emergencies (Sage).
However it is only an estimation and it is usually out of date because it relies on testing, admissions and death data. So there will always be a 14 to 21 day lag because people are infected weeks before they enter hospital, or die.
Contact pattern surveys are also used to calculate how much people move around but these also tend to have a lag of around a week and are open to bias because they rely on self-reported behaviour.
Household longitudinal surveys — which sample the same people repeatedly — allow a direct estimate of the infection rates, but these have only recently started by the Office for National Statistics (ONS) and no long-term data is currently available.
It is also crucial to look at the R value alongside the actual number of people infected.
The Chief Scientist of the World Health Organisation (WHO) Dr Soumya Swaminathan also said she did not think reopening schools would pose a threat of increased infections.
Speaking on the BBC’s Andrew Marr Show Dr Swaminathan, who specialises in paediatrics, said: “Children don’t seem to be getting seriously ill from this infection.
“We do know that children are capable of getting the infection, but there’s less data on how effectively they are able to spread it to others. What we have seen in countries where schools have remained open is that there have not been big outbreaks in schools.
“Where there have been it has been associated with events where a lot of people gather, not in regular classrooms and it’s often been associated with an adult, who has the infection and who has spread it. The risks to children are extremely low.”
Schools were closed in Britain on March 20 and a report published by Imperial College on March 30 suggested that might have taken the R rate down from three to just above two.
Prof Matt Keeling, Professor of Populations and Disease, at the University of Warwick, said: “Any form of additional mixing will inevitably lead to an increase in R, and hence more cases — however, all the current evidence suggests that children are not suffering substantially from this pandemic.
“This could be because they are less susceptible or because they show fewer symptoms when infected, there is no firm scientific data to determine which of these is true. This means that opening some school classes is unlikely to push ‘R’ above one; a conclusion supported by our mathematical modelling studies.
“The idea of only half the children being in school should mean that classes can be half the size, which will help with social distancing, further preventing a substantial rise in R.”
Should the R rate be used to guide policy?
Some experts believe the R rate should not be used at all to determine policy.
Prof Mark Woolhouse, Professor of Infectious Disease Epidemiology, University of Edinburgh, who also sits on Sage, said: “The R rate shouldn’t be a policy objective. It’s not the direct measure of the public health burden, it’s not a direct measure of capacity. It restricts your options very substantially.
“The R being below one for the whole country is meaningless when in fact when it can be different in different portions of the population.
“The public health implications of R greater than one are very different in hospitals than in the general population and the elderly and specific regions and having this single number directs policies.”
Prof Woolhouse said that the regional lifting of lockdown had been discussed by those advising the Government.
“It’s a discussion point, he said. “As I understand they prefer to move forward in the UK as one but the devolved administrations are doing things independently so there is an argument.
“There may be epidemiological circumstances where it would be justified to relax the region of the UK if the R value was lower.”
A Government source said: “We have always been clear that loosening depends on R being below 1.
“Without that, we risk seeing the kind of exponential growth we faced at the height of the pandemic. R must be below 1 for further opening up of society.”
Saul Faust, Professor of Paediatric Immunology & Infectious Diseases, University of Southampton, said: “Society has to reopen, children need to return to school as there are negatives for many of having to stay at home and we need to be able study transmission dynamics in all ages to help us learn how to manage this virus.”
How does R rate vary by region?
New figures from the Scientific Advisory Group for Emergencies released on January 22 showed that the South West, Midlands and Northwest of England have the highest R number: 0.9 to 1.2.
The regional figures have further revealed that the R number within the North East and Yorkshire is now between 0.8 and 1.1. In the South East, the value is between 0.7 and one, meanwhile in London, the R number is between 0.7 and 0.9.
However, experts are divided as to the relevance of the regional data, as it is unclear to what extent the different R values are driven by outbreaks in care homes and hospitals, which pose less of a threat to the average person than infections in the community.
The R is lowest in the East of England, at between 0.6 and 0.9. Across England, the R is 0.8-1.0.
The Department of Health and Social Care said the R is estimated to be below 1 in areas that have been under tighter restrictions the longest, such as Tier 4 over Christmas, which included London, the East of England and the South East.
However, scientists have warned that despite the reductions, case levels "remain dangerously high and we must remain vigilant to keep this virus under control, to protect the NHS and save lives".
Sage scientists said: "It is essential that everyone continues to stay at home, whether they have had the vaccine or not.
"We all need to play our part, and if everyone continues to follow the rules, we can expect to drive down the R number across the country."
In England, the percentage of people testing positive remained high but decreased slightly in the week ending 16 January 2021.
The ONS estimates that 1,023,700 people within the community population in England had COVID-19, equating to around 1 in 55 people, down from one in 50 from December 27 to January 2.
This is a snapshot of the average number of infections recorded by ONS taken at weekly intervals.
Would reopening schools affect the rate?
The other problem is that the R rate can vary not just regionally, but also by subsections of the population.
“Nobody catches it by passing someone on the street,” said one Government expert. “That isn’t how you get it. This is now epidemic of care homes and hospitals.”
There are fears that reopening schools will have a big impact on transmissions, potentially sending the R rate above one again, but there is little evidence to show that will happen.
Research published in April, by University College London (UCL), suggested that schools only play a small role in transmission, perhaps accounting for two to four per cent of deaths and 10 per cent of the R-rate.
Those estimates were based on the entire school population without extra measures, such as social distancing, so the impact is likely to be even smaller with class sizes restricted to 15 and communal tables replaced by traditional single desks.
There is also no evidence that those schools which stayed open for the children of key workers have seen extra infections in their communities.