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. 2010 Sep 2;6(9):e1000898. doi: 10.1371/journal.pcbi.1000898

Figure 2. Flow diagram for two compartment models of malaria transmission.

Figure 2

(A) shows the VSEIRS model and (B) shows the Inline graphic model. Human classes in (A) are Inline graphic (Susceptible), Inline graphic (Exposed), Inline graphic (Infected), and Inline graphic (Recovered). Mosquito classes are κ (latent force of infection) and λ (current force of infection). The possibility of transition between classes Inline graphic and Inline graphic is denoted by a solid arrow, with the corresponding rate written as Inline graphic. The average time of mosquitoes in the latent state is denoted by Inline graphic. The dotted arrows represent interactions between the human and mosquito stages of the parasite. The model in (B) adds clinical immunity [25], by differentiating between clinical infections that contribute to the measured cases, and less severe infections in a new class Inline graphic that are not clinical but remain infectious to mosquitoes at a lower level than Inline graphic. Clinical infections can fully recover becoming susceptible again, or remain parasitemic and transition to Inline graphic. Recovery from mild infections results in individuals who are fully protected from clinical disease, in class Inline graphic, whose further exposure to infected mosquitoes, can result again in mild infections. In time, clinical immunity can also be lost, with transitions from Inline graphic to Inline graphic, and therefore the return to full susceptibility. Only a fraction Inline graphic of individuals in Inline graphic contribute to the force of infection; the susceptibility to infection is reduced by a factor Inline graphic in class Inline graphic relative to Inline graphic.