Advertisement

What is the 'R0' value and why is it so important for the easing of the coronavirus lockdown?

What is the 'R0' value and why is it so important for the easing of the coronavirus lockdown? - TOLGA AKMEN/AFP
What is the 'R0' value and why is it so important for the easing of the coronavirus lockdown? - TOLGA AKMEN/AFP
Coronavirus Article Bar with counter ..
Coronavirus Article Bar with counter ..

There are a lot of figures involved in understanding the coronavirus pandemic, be it around cases, deaths or tests. But there is one number above all others that experts obsess over - and, increasingly, use to guide the UK government's approach to relaxing the lockdown.

It is called the reproductive value, or "R0 ", pronounced 'R nought' or 'R zero', and it basically tracks how many people, on average, will be infected for every one person who has the disease.

The number is not fixed. It can be altered by many things, including behaviour, which is why countries around the world have imposed stringent social distancing measures. Keeping people away from one another obviously makes a huge difference to the potential infection rate.

Since the lockdown, scientists in the UK estimate that the UK's reproduction value has dipped below one, and probably stands at around 0.7. This is key, because getting it below one is one of the government's five key tests for relaxing the lockdown. It means that every infected individual will pass the disease on to less than one other person, which ultimately means the epidemic dies out.

Aerial view of queues of shoppers maintaining distance as a preventive measure against Covid-19 outside a market in Peru - AFP
Aerial view of queues of shoppers maintaining distance as a preventive measure against Covid-19 outside a market in Peru - AFP

What is the R0 value?

The reproductive value describes the average number of people an infected individual can expect to pass the coronavirus onto. It is therefore a measure of how transmissible, or contagious, a disease is.

If a virus has an R0 of three, it means that every individual infected will pass the disease onto three other people if no containment measures are introduced.

“The reason we’re interested is because it not only gives you an idea of how many people the virus is likely to infect, but also an idea about how effective your interventions need to be to end the outbreak,” said Jonathan Ball, professor of molecular virology at the University of Nottingham.

What is the reproductive rate for Covid-19?

The World Health Organization estimated at the start of March that the R0 for coronavirus stands somewhere between 2 and 2.5. By comparison seasonal flu is estimated to be roughly 1.3 while measles has a reproductive value of between 12 and 18.

But, as discussed, these figures are not set in stone because the transmissibility of a disease is not based on biology alone.

“R0 is an indication of how much an infectious virus will spread in a population, and various things impact that value,” said Prof Ball.

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

Estimates of the R0 for Covid-19 still vary, partly because we still do not know how many people have been infected in total. According to modelling published by Imperial College London at the start of April, the R value stood somewhere between 3 and 4.6 in Europe before lockdowns came into effect.

In the UK now, the same team suggest that the UK's figure is between 0.5 and 0.75, although the figures in care homes may be different, and more testing would need to be done to have a more accurate picture.

It's also worth noting that the R0 is a measure of how infectious a disease is, but not how deadly.

What is the latest on the UK's R0?

The government is closely monitoring the UK's R0.

This week, Professor Chris Whitty, the UK's Chief Medical Officer, said it stood between 0.5 and one.

He said: "Obviously if it was lower, it would give us more room for manoeuvre, but it gives some room for manoeuvre, but it doesn't give a huge amount of room for manoeuvre.

"That's one of the thing ministers are going to have to consider - how to actually make sure we keep the "R" below one, given where we are at the moment thanks to what everybody is doing.

"But there isn't a magic number here. Lower is better but it's not that there is a specific number that has to be the right number."

Which interventions can help reduce the R0?

There are lots of infection control measures experts can use to push this number down and “flatten the epidemiological curve”. A study in the Lancet earlier this month, for instance, estimated that travel restrictions in Wuhan caused R0 to drop from 2.35 to 1.05 after just one week.

According to a pre-print study from the London School of Hygiene and Tropical Medicine published at the start of April, the average number of people an individual comes into contact with each day 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. The government's more recent estimates have since backed up this data.

So if it is working, why are not out of lockdown yet?

The R0 is not the only figure the government is tracking. It is also making sure that the pandemic does not overwhelm the NHS, looking for a "sustained and constant" fall in death rates, ensuring there is enough personal protective equipment (PPE), and finally, being confident that any changes do not risk another peak.

Prime Minister Boris Johnson said the UK is close to achieving these aims - but the final test, preventing a second wave of coronavirus, comes back round to the R0 again.

On Monday, Professor Whitty said that it was the job of the government's Scientific Advisory Group for Emergencies (Sage) to inform the government the effect on the R value of every decision taken on lifting the lockdown.

In Germany, where some lockdown measures were lifted once the R value made it to 0.7 - with some shops reopening, for example - R0 has crept back up to around 0.76, the head of the country's Robert Koch Institute on said on Thursday.

As such, its government plans to keep many social distancing measures in place for a longer period than initially expected. It is likely that the UK, too, will have to walk a similar tightrope, lifting some restrictions while keeping a close eye on R0 and cases, and adapting its plan depending on what happens.

This could mean many more months before things go completely back to normal, or, as Scotland's First Minister Nicola Sturgeon put it on Wednesday night: "People talk about lifting the lockdown, that is not going to be a flick of the switch moment - we're going to have to be very careful, very slow, very gradual.

"I'm far from convinced at this stage that when we get to the next review point on the 7th of May we'll be in a position to lift any of these measures right now, because the margins of manoeuvre that we're operating in right now are very, very, very tight and narrow."

Protect yourself and your family by learning more about Global Health Security