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14 May 2020updated 26 Jul 2021 8:36am

Why the much-discussed “R” is not the magic number

Ministers thought the reproductive rate of Covid-19 would be a useful measure to guide us out of lockdown – but the reality is not so simple. 

By Samuel Horti

On 22 April, England’s chief medical officer Chris Whitty explained how ministers would decide the next steps for leaving lockdown. The process centred on the R value, or reproduction rate of the coronavirus – the number of people somebody with the disease will go on to infect. If R rose above 1, the virus could spread exponentially, he said. 

The government’s scientific advisers would calculate the impact of different policies on R, package them in bundles that would each keep the number below 1, and present them to government. Ministers would then pick a path to follow. “There are a large number of possible combinations of things you can do,” Whitty said. “The question is: what is the best package?”

Over the past week, it has become clear that approach has been abandoned, and may in fact be unworkable.

First, R is tricky to calculate. Professor John Edmunds, who sits on the all-important Scientific Advisory Group for Emergencies (Sage) as well as Sage’s modelling subgroup, told MPs on the science and technology select committee last week that researchers around the country calculate R in different ways. Some examine death rates to work out how fast the virus is spreading; others – including Edmunds – favour analysing behavioural changes, such as patterns of contact between people. 

Every source is “biased in some way”, Edmunds said, and the scientists in the Sage sub-group, called SPI-M, meet twice a week to discuss their findings and reach a consensus. “It’s actually quite a difficult thing to get a handle on that 1 overall R,” he explained, adding that we simply do not have enough information to accurately calculate R in either hospitals or care homes.

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Edmunds’ London School of Hygiene and Tropical Medicine (LSHTM) colleague Professor Graham Medley, who also sits on both the main Sage group and SPI-M, told the BBC on Tuesday that we will likely “never know exactly” what R is. It’s definitely below 1, “but how far below 1 is a much more difficult thing to estimate,” he said. “In a sense, we’ve always been in that position. We will never know exactly what that value is.”

If calculating R is hard, predicting the impact of hypothetical policies on it is even harder. On Monday, the government floated the idea of two households forming a social “bubble” to meet one another. “There’s a lot of discussion about bubbles at the moment, and linking households up, keeping them together,” Edmunds said last week. “We’re quite uncertain about what would be the potential impact on those sorts of measures.” 

Asked whether scientists could predict the effect of allowing groups of, say, a dozen people to meet for parties, he described Sage’s models as “crude”. “We can look across regions… but beyond that, you’d have to have such a large-scale survey in order to go at an any more local level.”

Is it possible to calculate whether lifting a specific lockdown measure will keep the R value below 1? “No,” says Dr Marc Baguelin, of LSHTM and Imperial College London, who also sits on SPI-M. “I don’t think we are able to say that with any certainty.”

As Baguelin explains, R is still important, but policy decisions require a broader view. “It is important to understand how this R breaks down,” he says. “What is sustaining transmission? Is it ongoing transmission in hospitals? Care homes? More vulnerable groups in the population? Low adherence to recommendations? I think these questions are the important ones for a policy position. Even if the R is an important indicator, it is important to get it in perspective.”

For evidence ministers have abandoned the approach Whitty set out last month, look to the government’s own pandemic planning document, which says that R is “between 0.5 and 0.9 – but potentially only just below 1”. Edmunds told MPs that R is “between about 0.75 and 1, so just below 1”. If keeping R below 1 was ministers’ main concern, they would have given companies more than 48 hours to implement new coronavirus safety rules, some of which require new equipment, or even major layout changes. If, as Boris Johnson insists, lockdown decisions are driven by “data and public health” rather than “economic necessity”, the government would not be asking whole industries to return to work today. 

Ministers should be honest about how they are lifting lockdown. R is in the mix, but so are economics, the mental wellbeing of the public, old-fashioned politics, and, crucially, the incidence of coronavirus in the population (in Germany, R has been above 1 for four days, but scientists aren’t worried because the level of disease is so low). R should be a measure of our success – not the goal itself. Despite what Whitty said last month, R is not the magic number some have made it out to be.

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