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Covid-19: What is the R number?

BMJ 2020; 369 doi: (Published 12 May 2020) Cite this as: BMJ 2020;369:m1891

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  1. Elisabeth Mahase
  1. The BMJ

It’s been driving policy decisions since covid-19 emerged in late 2019, but what is the R number, and does it matter, asks Elisabeth Mahase

What is the R number?

The R number could refer to either the basic reproduction number, known as the R nought or zero (R0), or the effective reproduction number (Re).

R0 describes how many people each infected person will infect on average, assuming that there is no pre-existing immunity in the community. It is often estimated using three factors: the duration of contagiousness after a person becomes infected, the likelihood of infection in each contact between a susceptible person and an infectious person or vector, and the frequency of contact.

Re is the number of people that can be infected by an individual at any specific time, and it changes as the population becomes increasingly immunised, either through individuals gaining …

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