Fig 8. Susceptible-infected-susceptible (SIS) compartmental model representing the dynamics of malaria in a heterogeneous host population considering asymptomatic infections.

The compartments correspond to the following epidemiological classes: S_i,j represents susceptible individuals from risk group j (1 = low-risk [LR]; 2 = high-risk [HR]) who have experienced i clinical malaria attacks; I_i,j represents individuals with symptomatic infection from risk group j who are currently experiencing their ith clinical malaria attack; A_i,j represents individuals with asymptomatic infections from risk group j with i past clinical malaria attacks. Individuals experience malaria episodes due to an age-dependent force of infection λ(a) modified by a risk factor x_j, and a partial immunity weight σ(i). Individuals from compartments I and A recover and become susceptible again at rates γ and γ′, respectively.