Figure 4.

Effect of seasonality on the transmission potential of P. vivax. (a) For non-relapsing P. falciparum, R₀ decreases with increasing seasonality. For tropical strains of P. vivax, there is a modest reduction in R₀ with increasing seasonality. By contrast, for temperate strains of P. vivax, R₀ increases with seasonality. In the simulations presented here, the difference between R₀ for P. falciparum and P. vivax is owing to relapses. (b) The transmission potential of temperate strains of P. vivax will depend on the duration of dormancy, with R₀ optimized at longer durations in more seasonal settings. (c) For a given seasonal profile, the time to first relapse (dormancy plus latency in the liver) can be estimated by maximizing R₀. The time to first relapse (solid line) is predicted to increase with seasonality. Notably when seasonality crosses a threshold of ≈50% of transmission in the peak three months, the time to first relapse switches from 3–4 months characteristic of tropical phenotypes [24,38,39] to 6–9 months characteristic of temperate phenotypes [27,40].