Health

What is the ‘R’ value and why is number one critical to coronavirus fight?


“R VALUE” is a phrase Brits have heard time and time again when scientist and those alike refer to coronavirus figures and scientific statistics.

But what does it actually mean and why do scientist keep using the term.

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 Scientists have said that the R value overall is below one in the UK

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Scientists have said that the R value overall is below one in the UKCredit: Getty Images – Getty

During the Downing Street press conference on Thursday, April 16, Sir Patrick Vallance, the Government’s Chief Scientific Officer, said the R value overall is below one – which shows Brits can expect the slowing and the turn of the epidemic.

He said: “Across many regions, the numbers are decreasing…this is consistent with the idea that the R is below one, somewhere between 0.5 and one.”

But as this term is thrown around by experts and ministers, what does the R value actually mean and why does it matter?

What is the R value?

R0, or R nought, refers to the average number of people that one infected person can expect to pass the coronavirus on to.

Scientists use it to predict how far and how fast a disease will spread – and the number can also inform policy decisions about how to contain an outbreak.

For example, if a virus has an R0 of three, it means that every sick person will pass the disease on to three other people if no containment measures are introduced.

It’s also worth pointing out that the R0 is a measure of how infectious a disease is, but not how deadly.

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What does it mean for Covid-19?

The World Health Organisation (WHO) estimated at the start of March that the coronavirus R0 stands somewhere between 2 and 2.5.

In comparison, the seasonal flu is estimated to be roughly 1.3 while measles has a reproductive value of between 12 and 18.

Despite this, these figures are not set in stone because a given pathogen’s R value changes with place and time.

Jonathan Ball, professor of molecular virology at the University of Nottingham, told The Telegraph: “R0 is an indication of how much an infectious virus will spread in a population, and various things impact that value.

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“The susceptibility, size and density of the population that the infection is introduced into matters, as well as the infectiousness of the virus itself.”

Predictions of the R0 for Covid-19 are currently varied because no one knows exactly how many people have been infected in total.

According to modelling published by Imperial College London, the R value stood somewhere between 3 and 4.6 in Europe before lockdowns came into effect.

What is the R value in the UK?

On Sunday, May 10, 2020, Boris Johnson urged the British public to help reduce the R value.

In his address to the nation, he said: “It depends on all of us – the entire country – to follow the advice, to observe social distancing, and to keep that R down.”

He added: “We must make sure that any measures we take do not force the reproduction rate of the disease – the R – back up over one, so that we have the kind of exponential growth we were facing a few weeks ago.

“And to chart our progress and to avoid going back to square one, we are establishing a new Covid Alert System run by a new Joint Biosecurity Centre.

“And that Covid Alert Level will be determined primarily by R and the number of coronavirus cases.”

He continued: “And in turn that Covid Alert Level will tell us how tough we have to be in our social distancing measures – the lower the level the fewer the measures.”

The higher the level, the tougher and stricter we will have to be.

During one of the Downing Street press conferences, Sir Patrick revealed that it was “highly likely” there is an R value in the community of less than one.

This means that every infected individual passes the disease to less than one other person.

This shows the lockdown has had a positive impact on the coronavirus outbreak as to bring an outbreak under control the R0 value needs to fall below one.

However, when the number remains higher than one, the epidemic will grow.

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Sir Patrick admitted that the R0 value could be higher in some care homes and hospitals.

He said: “As I’ve said, it’s not true that the R is necessarily below one in every hospital or in every care home, and that’s the important area that we now need to look at and make sure that the appropriate measures are in place to try and reduce the R there.

“But it doesn’t change the overall view that I’ve described, that the R overall is below one and therefore we expect to see the slowing and the turn of the epidemic.”

What can reduce the R value?

There are lots of infection control measures experts can use to push this number down and reduce the spread.

A study in the Lancet in April, for example, estimated that travel restrictions in Wuhan caused R0 to drop from 2.35 to 1.05 after just one week.

Sir Patrick said that the draconian social distancing measures introduced in the UK have had a substantial impact so far.

This is because as less people come into contact with one another, there is less chance for the virus to spread.

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He said on Thursday, April 16: “The social distancing measures are needed to reduce the levels right down to a low level.

“At that point there may be decisions about which ones to relax and which ones not to relax.

“It’s important to keep the R below one, and this is all about trying to reduce contacts, particularly between households, reduce transmission and keep the levels low across the community.”

 

And Sir Patrick added that even small changes in the measures that are in place “could lead to the R going above one.”

According to a pre-print study from the London School of Hygiene and Tropical Medicine, the average number of people an individual comes into contact with each day has dropped by 73 per cent since the UK’s lockdown began.

“This would be sufficient to reduce R0 from a value from 2.6 before the lockdown to 0.62 during the lockdown, indicating that physical distancing interventions are effective,” the study, which tracked over 1,300 adults and has not yet been peer reviewed, concluded.

Sir Patrick Vallance says coronavirus infections are falling in the community but transmission in care homes is less clear





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