“The basic reproductive number, R0, is widely used in infectious disease research to estimate the transmission potential of a disease. R0 is defined as the average number of secondary infections caused by a single infection, assuming that the population is completely susceptible and that there are no interventions. Generally, a disease with an R0 value < 1 will eventually die out, while a disease with an R0 value > 1 may continue to spread and trigger an epidemic. Average R0 values for some common infectious diseases are listed in Table 1. In its simplest form,
R0 = pcd
where p is the transmissibility of the disease, c is the rate of contact with uninfected members of the population, and d is the duration of contagiousness. In reality, a population may not be entirely susceptible and interventions may provide immunity to certain members of the population. Thus, the effective reproductive number, R, is a means of relating R0 to the actual level of susceptibility and is given by
R = R0x
where x is the proportion of the population which is susceptible. One highly successful method of combating an outbreak is vaccination. The proportion of the population, V, which would need to be vaccinated in order to reduce the effective reproductive number to < 1 is
V = 1 – (1/ R0)
Estimates of R0, R, and V are used to determine the appropriate level of intervention necessary to control infectious diseases.
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