Four months ago, 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 – is only going to become more important over the coming months. It’s so important that the government dedicated a sizable chunk of its daily press briefing on April 30 to explaining R and detailing how closely this value will determine how and when the UK moves beyond its current lockdown state.
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 was 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.
Read more: The UK’s lockdown roadmap and rules, explained
Why is R so important?
The UK government placed a strong emphasis on R in its press briefing on April 30, going as far as playing a two-minute video that sketched out the importance of this number. As Chris Whitty, the chief medical officer for England, said at the briefing, “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 easing the current lockdown, government advisors will be keeping a very 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. As Boris Johnson said at the April 30 press briefing, the government’s current aim is avoiding a second peak of the disease, and that means keeping the R consistently below one.
In a televised statement on May 10, Boris Johnson set out the future of the UK's lockdown measures. The government is introducing a new Covid-19 Alert Level that will take into account both the current R number and the total number of infections in the UK to set the overall risk level, although the government did not detail how exactly this level would be calculated. A government Covid-19 recovery strategy released on May 11 detailed that plans to re-open shops, restaurants and bars would hinge partly on an assessment of the transmission rate of the virus.
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.
To get a more up-to-date sense of the coronavirus R number, the UK is starting to test random samples of the population to see how many of them have the disease. This test, which will be carried out monthly, will involve at least 25,000 people who will be tested to see if they’re currently ill with the virus. This should help the government track the changes in R over time and see if the infection rate is rising or falling.
What is R right now?
At the April 30 press conference, the UK’s chief scientific officer Patrick Vallence said that the UK’s R0 was between 0.6 and 0.9 while the figure in London was between 0.5 and 0.7. It’s worth noting that the precise R will vary from location to location, but these numbers suggest that the enforced lockdown is having the desired effect of forcing the transmission of the virus down.
What isn’t clear, however, is how the R will change if and when the government chooses to relax its social distancing measures and it’s this change that advisors will be keeping a very close eye on to see if the UK is keeping a second peak at bay. It’s worth noting that the R won’t be the only number worth keeping an eye on. Also critical is that the UK doesn’t exceed its critical care bed capacity, and that the NHS has enough spare capacity to resume normal treatments as this too will have a big impact on the overall mortality of the coronavirus pandemic.
Updated 11.05.2020: The article has been updated to include details from the prime minister's televised speech on May 10[/i]
Matt Reynolds is WIRED's science editor. He tweets from @mattsreynolds1
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This article was originally published by WIRED UK
