Fig. 3.

Consequences of alternative assumptions about serotype interaction for dengue persistence and eradication. (A) Critical community size. Comparison of results from stochastic realizations with monthly DHF incidence from Thai provinces (Inset). Simulations where there is strong ADE (χ = 3) and virtually no cross-immunity (δ = 365 per year) demonstrate high extinction probabilities over all population sizes ≤2 million. Simulations where there is moderate ADE (χ = 1.5) and a 4-month period of cross-immunity (δ = 3 per year) show decreasing extinction probabilities as the population size increases above 1 million; consistent with the Thai data (7). (The black line denotes the least-squares best-fit exponential curve.) Other model parameter values are as in Fig. 1, and results are averages of 50 realizations with standard error bars. (B) Effects of ADE and difference in virulence on vaccination thresholds. If two serotypes have slightly different R₀, then the serotype with the smaller R₀ is either more or less difficult to eradicate in the presence of the other serotype depending on whether there is ADE or increased mortality from the other serotype. Critical vaccination level is given by the following expression, 1 − ((R₀₂/R₀₁ − 1)/c + 1)/R₀₂, where c ≈ χ(1 − ρ₂).