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A, or the “effective reproduction number”, is a way of rating a disease’s ability to spread. It’s the average number of people on to whom one infected person will pass the virus. For an R of anything above 1, an epidemic will grow exponentially. Anything below 1 and an outbreak will fizzle out – eventually.
At the start of the coronavirus epidemic, the estimated R for coronavirus was between 2 and 3 – higher than the value for seasonal flu, but lower than for measles. That means that each person would pass it on to between two and three people on average, before either recovering or dying, and each of those people would each pass it on to a further two to three others, causing the total number of cases to snowball over time.
The reproduction number is not fixed, though. It depends both on the biology of the virus, but also our behavior, such as social distancing. It also depends on the population’s immunity. At the start of an outbreak scientists often refer to the “basic reproductive number”, R0, which is a similar measure to R, but starts with the premise that the population has zero immunity through infection or vaccination.
Under the current lockdown restrictions, R is now estimated to have dropped to somewhere between 0.6 and 0.9. This means that the epidemic is shrinking – although not uniformly. The figure given for London is 0.5 to 0.7, with some parts of the UK having a higher or lower figure.
There is no obvious answer for what R needs to be in order for the lockdown to be lifted. That depends on how low ministers want the total number of cases to be if the government is to be able to effectively track and trace new cases. It is also not clear precisely how easing different restrictions will influence R – there is still uncertainty about how much schools contribute to transmission, for instance. So real-time tracking of transmission will be needed to continue providing a rolling estimate.
Because once R inches above 1, even just a fraction, the number of cases will start sharply rising again, putting the UK on the path to a substantial second surge.
