Some comments on the reproduction number of the virus (“R” or “R0“) after the figure was mentioned in the Government’s daily press conference earlier this evening.
Prof Rowland Kao, Sir Timothy O’Shea Professor of Veterinary Epidemiology and Data Science, University of Edinburgh, said:
“The reproduction number of the disease is an estimate of the current average rate at which infected individuals cause new infections. While the exact way of determining R depends on how the disease is transmitted, fundamental to all definitions is that when R is below one, the disease is in decline.
“An important distinction between R and R0, which is the basic reproduction number, is that R0 is the value of R when people have not yet been exposed to infection. If R0 is below one then a disease can be expected to die out on its own without intervention and before it can infect a large proportion of the population. In contrast, R changes over the course of an epidemic, usually becoming smaller as controls take effect and the number of susceptible people remaining goes down. Importantly, if controls are relaxed, or the disease ‘jumps’ to a new location where fewer people have been exposed, or where there are fewer controls, R can go up again, causing new waves of infection.”
Dr Robin Thompson, Junior Research Fellow in Mathematical Epidemiology, University of Oxford, said:
“The reproduction number, or “R”, represents the number of individuals that a typical infectious person is likely to go on and infect. The value of R changes during an outbreak in response to public health measures, and is currently below one. However, if current measures are relaxed, the contact rate between individuals will increase, potentially leading to more infections. As a result, there would be a danger that the reproduction number increases back above one and we see a second wave of infection, as well as a second peak.
“A key challenge now for epidemiologists is to identify measures that can be relaxed that have only limited impacts on the value of R. One of the reasons that this is particularly challenging is that interventions were first introduced in the UK within a few days of each other. As a result, it is hard to disentangle the relative effects of different interventions on the reproduction number.”
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