## The Effective Reproductive Number

The effective reproductive number, R, is a measure of how many new infections on average result from each existing infection (Anderson and May, 1991); R is also known as the effective reproductive ratio or, incorrectly, as it is not strictly a rate, the effective reproductive rate. When R = 1, on average each infection produces only one further infected individual. If there are sufficient numbers of infections in the population to ensure that it does not die out simply through chance effects, the infection will be at equilibrium in the population and if this equilibrium is self-sustaining the infection will be endemic; where on the other hand R is significantly greater than one, a growing number of infections, i.e., an epidemic, will result. Although R is a useful measure of the speed with which an infection in a population is spreading, it changes over time as infection spreads through the population and does not give an indication of the potential rate of increase of infection in other populations because the value of R is governed not only by the characteristics of the infection itself but also by the proportion of the population which is immune through past infection or otherwise.

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