Evolution and emergence of novel human infections

Arinaminpathy, N.; McLean, A. R.

The basic reproductive number, R 0 , is the average number of secondary cases arising from a single infected case, in an otherwise susceptible population (Anderson & May 1991) . It is a measure of emergence potential, as R 0 . 1 is a necessary condition for emergence. It is, however, not a sufficient condition, as such pathogens are subject to stochastic extinction with probability 1/R 0 n , where n is the number of index cases (May et al. 2001) . Conversely, infections with R 0 , 1 stutter to extinction with probability 1. Moreover, different regimes for R 0 , 1 show differing epidemiological behaviour in terms of the numbers of cases that outbreaks could involve. Figure 1 shows the probability distributions for outbreak sizes for R 0 ¼ 0.1 (a poorly adapted pathogen) and R 0 ¼ 0.9 (an almost-adapted pathogen). Although neither pathogen is capable of emergence, it is evident that a pathogen with R 0 ¼ 0.9 is capable of causing much larger outbreaks than one with R 0 ¼ 0.1. Indeed, the mean outbreak size for R 0 , 1 is given by 1/(1 2 R 0 ) (Becker 1974) .