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The R Number

This time last year, the phrase “R number” didn’t mean much to anyone except epidemiologists and infectious disease experts. But as the coronavirus pandemic has raced around the world, infecting millions and killing hundreds of thousands, just keeping up with the news is starting to feel like taking a crash course in infectious disease epidemiology.

This once-obscure term – R – has become important over the last year. It’s so important that the government dedicates a sizable chunk of its daily press briefings to explaining R and detailing how closely this value will determine how they attribute tiers.

Here’s everything you need to know about the figure at the centre of the government’s pandemic planning.

What is R?

At its core, R is pretty simple. It stands for “effective reproduction number” and is a way of measuring how a disease spreads through a population. If the R of a disease is one, then on average every person infected with the disease will go on to infect one more person.

If the R is higher than one, then that means that a disease will keep on spreading to more and more people. Imagine that coronavirus had a R of two. That would mean that every person with the disease would go on to infect two new people. So, if you started with 100 infected people, they would infect 200 people who would then go on to infect 400 people.

Even if the R were a lot lower – say 1.2 – the disease would still move through a population really quickly. Those 100 infected people would infect 120 people, who would then infect 144, then 173, then 208. In just four rounds of transmission, the number of people each time would double. Mathematicians call this phenomenon exponential growth, and it explains why coronavirus infected so many people all over the world so quickly.

But the opposite is true too. If the R is below one, then an epidemic will eventually fizzle out altogether. If R was 0.7, then 100 infected people would go on to infect 70 people, who would go on to infect 49, then 34 and so on. That’s why anything over one is bad news, but the further under one R goes, the better off we are.

What does R depend on?

R isn’t a fixed number it changes over time and from place to place. Factors that can shift the R of a disease include changes in the biology of the disease as well as behavioural decisions that humans make, such as implementing social distancing. It’ll also vary in different locations, so if coronavirus took hold in a densely-populated city that hadn’t experienced an outbreak and didn’t have any social distancing rules in place, it’d likely have a much higher R than in a place where social distancing had been implemented for a long time.

At the start of the coronavirus outbreak scientists were busy calculating the virus’ basic reproduction number: R0, which is pronounced “R naught”. This describes how infectious a disease is in a population that has never been exposed to the pathogen before so has no immunity at all. Initial studies on the transmission of coronavirus in China put the R0 at somewhere between two and 2.5.

To put that in perspective, seasonal flu has an R0 of roughly 1.3 while measles is much higher, at between 12 and 18. But in both of those cases, the actual R is much lower because we have vaccines. Obviously with coronavirus that is not the case, although if people do gain immunity after exposure to coronavirus that’s one way that R may be driven down in some populations.

Why is R so important?

The UK government places a strong emphasis on R.  As Chris Whitty, the chief medical officer for England, said, “as soon as R goes above one, you restart exponential growth.” If that growth goes on unchecked, cases and hospitalisations will start to rise very quickly, and the UK will be back in the situation it was facing in late March with the NHS at risk of being overwhelmed with cases.

As the UK government starts to consider ways of reducing the tier in some areas, government advisors will be keeping a close look at the R of the outbreak. If it starts heading up, it could mean that the relaxed social distancing measures are increasing the rate of transmission. If it goes too high, then the government may have to consider tightening restrictions again.

How do we measure it?

There are lots of different ways of calculating R. Scientists have been looking at the genome of the virus to see how it changes as it infects new people. They can also look to death and hospitalisation figures to get a sense of how many people have contracted the disease. The problem is that most of these methods involve looking into the past to some degree. People who unfortunately die from coronavirus will have been infected weeks before, so using that data to calculate R can only tell you how widely the disease was infecting people several weeks ago.

What is R right now?

The reproduction rate R in Scotland is currently estimated as being between 0.7 and 0.9.</p

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