Figure 2.

Probabilities, as a function of introduction time, that a significant outbreak (at least 50 simultaneous infections) results from the introduction of one individual infected with a mutant strain into a population experiencing a wild-type epidemic. (a) Non-seasonal transmission; (b) seasonal transmission. Circles, black: mutant strain has no antigenic advantage (σ = 0); triangles, mid-grey: mutant can re-infect individuals immune to wild-type with probability σ = 0.1; squares, pale grey: σ = 0.3; plus symbols: σ = 0.7. Each set of points is based on 10 000 independent trials with random introduction times. Initially all hosts susceptible except for 10 individuals with wild-type infections. Parameter values as in electronic supplementary material, table S1.