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reproductive rate (symbol R0) has proven a very useful (and comprehensible)
measure of the transmissibility or spreading potential of an infection in a population.
It is R0 which is actually illustrated in slide 5 (where R0=3).
R0 is an abstraction: no population actually experiences spread of an infection at this rate (with the exception of the extreme situation of an isolated area, totally virgin to a microorganism, where an infectious case enters).
R0 is independent of the number (or proportion) of individuals susceptible or immune in a given population.
R0 is determined by basic biological features of the microorganism and of man as well as by social characteristics and public health practices in the population which relate to the risk of a potentially transmitting contact between individuals. R0 is, therefore, specific to a microorganism and a population.
(The definition of R0 presented here is taken from: Giesecke J. Modern infectious disease epidemiology. London: Arnold, 1994.)