Models of Acquired Immunity to Malaria: A Review

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episodes (see [115]). This suggests that the rate at which the immune population become

susceptible again, γ, is dependent on the rate at which they acquire new infections, h [38].

However, the average number of years without exposure that can lead to loss of pro-

tective immunity is yet unclear and it’s known to vary with individuals. A related study

reveals that emigrants from Africa to Europe loose much of their immunity after over 2

to 3 years of their stay while remaining protected from severe disease attacks [91]. The

outbreak of malaria infection in the central highlands of Madagascar aided the opportunity

to compare malaria incidence in first-time exposed children and young adults, with that in

older adults who spent their childhood in the study area before the introduction of malaria

control measures [215]. It was found that, individuals older than 40 years were more pro-

tected than younger adults. This increased protection was suggested to be probably due to

immunological memory. These works are consistent with the evidence that fatality rates are

significantly higher among non-immune individuals exposed to malaria for the first time

than among previously immune humans revisiting endemic areas (see [76]), because prior

exposure has a considerable protective effect [131], [130], [192]. This also suggests that

clinical immunity may hardly be completely lost as most deterministic modellers assume

[215]. With the assumption that mortality acts approximately equally on all individuals,

the average per capita rate of losing immunity is given in [45] as

γ =

he^−hτ

1−e^{−hτ .}

(5.1)

The changes in γ with respect to h and τ, in the absence of re-exposure, are presented

in Figure 5.3. The model demonstrated that with a high force of infection as a result of

continuous exposure, the rate of losing immunity could almost get to zero but appreciable

rates of immunity loss are noticeable with lower force of infection and short durations

Figure 5.3: Changes in the rate of immunity loss γ with different values of h and τ.