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Training Manual on Wildlife Diseases and Surveillance
The value of R distinguishes a population that can maintain infection with a particular
pathogen over time (R>1) from populations which cannot maintain a pathogen in the long term
(R<1). For example, measles in people generally has a very high R value when the virus is
introduced into a susceptible population. R0 for measles is usually about 18! (Each infected
individual on average will infect 18 other people before the individual dies or recovers from the
infection). However, R quickly drops as the virus spreads rapidly through the population and in
small human populations, measles dies out quickly. It is estimated that, to maintain measles
virus, a human population of 300,000 to 500,000 in-contact individuals is required. In human
populations of this size, there are enough births of new, susceptible individuals to permit
pathogen transmission to continue and the virus to be maintained in the population.
R is very hard to measure directly. It usually is estimated by measuring a variety of other
parameters over time and then using complicated mathematical formulas to estimate R. There is
a scientific literature concerning R and its measurement or estimation, and thus there is good
reference material available to guide those who need to estimate R for a pathogen in a particular
host population and environment.
Estimates of R have great value in disease management programmes. R is especially
valuable for planning vaccination campaigns aimed at eliminating a pathogen or reducing its
impact. The objective of a vaccination programme is to increase the proportion of immune
individuals in a population sufficiently to cause R to become less than one (R<1), and thus to
eliminate the pathogen from the population or greatly reduce its impact. If R can be estimated
accurately for a given pathogen in a particular population, it is possible to estimate what
proportion of the population must be vaccinated to achieve (R<1). The formula is:
Minimum proportion that must be vaccinated = 1-(1/R)
Small pox virus was eliminated from humans globally by vaccination campaigns based on
careful estimates of R.