How the mighty R rate has lost its starring role

Through much of the pandemic it has seemed ministers and scientific advisers could barely fashion a sentence without mentioning the “R”.

A hitherto obscure epidemiological concept which describes whether a disease is spreading or in retreat, the term was referenced incessantly.

An R of less than 1, denoting that each infected person transmits the virus to less than one other, was even made an explicit condition of the official strategy to exit lockdown.

As with much of the Government’s core messaging, the idea successfully cut through. Ordinary people began to speak of the R as though discussing the weather or a football score.

It is striking, therefore, that amid the public statements in response to the surge of cases in Leicester, and the subsequent local lockdown, the R value has scarcely been mentioned.

Addressing the Commons on Monday, Matt Hancock, the Health Secretary, spoke of the seven-day infection rate and of cases per 100,000 (Leicester has 135 – three times that of the next highest city). He also talked of daily hospital admissions for Covid-19 in the area (between six and 10 in Leicester, against one at other trusts).

Rather than a volte-face, Mr Hancock’s choice of jargon reflects a growing discomfort with the R among policymakers. As far back as the second week of June a senior government scientist in an off-therecord briefing, issued a “word of caution” to journalists about the term, particularly when applied regionally.

He explained that as new case numbers came down, the R rate painted a less and less nuanced picture. The following week, the Government began publishing “growth rates” alongside R values.

For example, while the R for the UK on that day (June 19) was 0.7 to 0.9, the growth rate was between minus 4 and minus 2 per cent. “R estimates do not tell us how quickly an epidemic is changing and different diseases with the same R can result in epidemics that grow at very different speeds,” a spokesman explained. “A disease with R=2, with infection lasting years, will grow much more slowly than a disease with R=2 with infection lasting days.

“Growth rates provide different information to R estimates, by informing us of the size and speed of change, whereas R only gives us information on the direction of change.”