As much as the government has any kind of strategy for lifting Britain out of lockdown it appears to revolve around the ‘R’ – or Reproduction – number. So long as this stays below one, we are told, the epidemic cannot progress – while the moment it strays above one then the disease will start to grow exponentially. That is easy enough to understand in itself. What is less easy to work out is just how this R number is calculated. We are told that for Britain as a whole it currently lies somewhere between 0.7 and 1. But whether this really means an awful lot is open to question. According to the government’s explanatory notes the number is a ‘consensus estimate’ from SAGE, the Scientific Advisory Group for Emergencies. It also partly relies on hospital data which is two to three weeks out of date.
There is a simpler and more up-to-date option, which is already being used in Germany and Ontario: link the lifting of lockdown measures to the number of infections per 100,000 population. Since 6 May, Germany has used a threshold of 50 cases per 100,000 people over the previous seven days. If a district or urban municipality stays below that level, the easing of lockdown measures can progress; if it exceeds it then restrictions have to be reimposed. Ontario, by contrast, has proposed a threshold of 200 new cases per day. With a population of 14.3 million this works out at just under 10 cases per 100,000 per week.
How would it work in England if we used one of these thresholds? If we used the German standard, every region would now be relaxing its lockdown. In the week to 15 May, the highest rate of new infections was measured in the North East, with 18.9 cases per 100,000 population. If we used the Ontario measure, two regions could now be relaxing their lockdowns: the South West, at 4.8 new infections per 100,000 and London at 5.1. The South East, at 10.7 and the East, at 11.4, would be close to being able to do so.
There is a weakness in using the German or Ontario approach: the number of new infections being measured depends very much on the number of tests being undertaken. It is a peculiarity of Covid-19 that while a small minority suffer serious, or mortal, illness, between 50 and 80 per cent of those infected, to judge by Italian and Chinese studies, could remain entirely symptom-free. Therefore, a country which embarks on mass testing of its population is going to show a much higher number of new infections per 100,000 than if (like Britain throughout much of this crisis) it only tests people who seek medical attention.
Perhaps, instead, we should seek to set a threshold for hospitalisations per 100,000 – set at a much lower rate than the German or Ontario thresholds. This is the suggestion of a group of Canadian infectious disease specialists in a letter to the National Post this week.
Compared with the R number it would have the benefit of being simple to understand and a lot more up to date.