In search of the R number
A few weeks ago, the phrase “the R number” didn’t “mean much to anyone except epidemiologists”, said Matt Reynolds in Wired. But as Boris Johnson made clear in his Sunday night address, the nation’s fate now depends on it. Lockdown policy will be determined mainly by “the R” – the virus’s reproduction number, or the average number of people to whom each infected person spreads the disease. If it is above one, a disease will grow exponentially; if the R is two, 100 people will infect 200, who will infect 400. But if it is below one, an epidemic will “fizzle out” in time. The R is now thought to be between 0.5 and 0.9. If it stays below one, all will be well. But if it strays above that, restrictions will tighten again.
“There’s just one problem with that,” said James Ball in The Spectator. The R is “a figure few people think we have any real ability to track day to day”. Epidemiologists work it out using mathematical models, based on available data about cases and deaths. But there are “lags built in at every stage” of the data. Hospitalisations happen several days after infection; deaths, days or weeks after that. Both may not be recorded for some time. The R changes every day, and varies greatly by region. Currently, transmission seems to be happening mostly in care homes, not in the community – so the national average R tells us “very little”.
When officials refer to the R number, it sounds definitive. But the impression of precision may be false, said Rachel Sylvester in The Times. Some briefings have descended into what the statistician David Spiegelhalter calls “number theatre”. The PM’s equation for the threat level – “C (the Covid alert level) = R (the rate of infection) + N (the number of infections)”, where C is between one and five; R is about one, and N is above 230,000 – is “gobbledegook”. True, but the R is still the best indicator available of how the virus is spreading, said Clive Cookson in the FT. For that reason, officials will continue to use it – “despite its imperfections”.