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What R You Talking about?

What is an R number, and can we trust it? Why does this matter? Our resident scientist Niamh Mortimer breaks down the science.

A recent study by Imperial College showed that there were 13 positive coronavirus disease 2019 cases per 10,000 people. This means the UK has an R number of 0.57 – a lot lower than the 0.7 previously estimated a month ago.


R0 is the basic reproduction number, also known as the basic reproduction rate. It tells us how contagious an infection is at the current moment and is affected by many different factors such as how often people come into contact with each other, how likely it is the virus will be passed on during contact and how long a person is infectious for.

There are plenty of other biological, socio-behavioural, and environmental factors that can affect R0 including population density, seasonality, and social organisation.

If R0 is bigger than 1 , then the infection outbreak is expected to continue spreading.

If R0 is less than 1, then the infection outbreak is expected to come to an end.

R0 can be used to guess the number of people that must be vaccinated in order to remove the infection from the whole population.

R0 is not related to time – it does not tell us whether new cases will occur with the next day or months later. It also does not tell us the severity of an infectious disease or how quickly it can spread. Instead, R0 is most accurately described in terms of ‘cases per case’.


R0 is an estimate of how contagious a disease outbreak is, based on human behaviour and biology of the pathogens.

Currently the R0 for coronavirus disease 2019 in the UK is less than 1, suggesting the outbreak is going to decrease.


R0 has to be estimated, reported, and applied with great caution because it is not simple.

There are several different definitions for R0 including but not limited to: ‘the number of secondary cases one case would produce in a completely susceptible population’ or ‘average number of secondary cases’ or ‘expected number of secondary cases’ – this can cause confusion and may result in many different R0 numbers being calculated for the same outbreak.

Many researchers calculating R0 have not been trained in specific mathematical techniques, making R0 often misrepresented, misinterpreted, and misapplied.


Estimated: roughly calculated.

Contagious: spread of a disease from one person to another.

Pathogens: bacteria, virus or other organism that can cause disease.

Susceptible: someone who is likely to catch a disease.


Delamater PL, et al. Complexity of the basic reproduction number (R0). Emerg Infect Dis, 2019; 25(1): 1–4.

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