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Age-shifting in malaria incidence as a result of induced immunological deficit: a simulation study.

Pemberton-Ross P, Smith TA, Hodel EM, Kay K, Penny MA - Malar. J. (2015)

Bottom Line: Predicted shifting of burden continue into the second decade of the programme.Conversely, short-lived transient changes in incidence measured soon after introduction of a new intervention may give over-positive views of future impacts.Complementary intervention strategies could be designed to specifically protect those age-groups at risk from burden shift.

View Article: PubMed Central - PubMed

Affiliation: Swiss Tropical and Public Health Institute, 4002, Basel, Switzerland. peter.pemberton-ross@unibas.ch.

ABSTRACT
Effective population-level interventions against Plasmodium falciparum malaria lead to age-shifts, delayed morbidity or rebounds in morbidity and mortality whenever they are deployed in ways that do not permanently interrupt transmission. When long-term intervention programmes target specific age-groups of human hosts, the age-specific morbidity rates ultimately adjust to new steady-states, but it is very difficult to study these rates and the temporal dynamics leading up to them empirically because the changes occur over very long time periods. This study investigates the age and magnitude of age- and time- shifting of incidence induced by either pre-erythrocytic vaccination (PEV) programmes or seasonal malaria chemo-prevention (SMC), using an ensemble of individual-based stochastic simulation models of P. falciparum dynamics. The models made various assumptions about immunity decay, transmission heterogeneity and were parameterized with data on both age-specific infection and disease incidence at different levels of exposure, on the durations of different stages of the parasite life-cycle and on human demography. Effects of transmission intensity, and of levels of access to malaria treatment were considered. While both PEV and SMC programmes are predicted to have overall strongly positive health effects, a shift of morbidity into older children is predicted to be induced by either programme if transmission levels remain static and not reduced by other interventions. Predicted shifting of burden continue into the second decade of the programme. Even if long-term surveillance is maintained it will be difficult to avoid mis-attribution of such long-term changes in age-specific morbidity patterns to other factors. Conversely, short-lived transient changes in incidence measured soon after introduction of a new intervention may give over-positive views of future impacts. Complementary intervention strategies could be designed to specifically protect those age-groups at risk from burden shift.

No MeSH data available.


Related in: MedlinePlus

OpenMalaria function modelling the effect of age on number of mosquito bites received. Shown is the ratio of bites received to those received by an adult  by age a(i, t). The continuous line corresponds to proportionality between bites received and expected body surface area. The dotted line shows a similar model which assumes proportionality between bites received and body weight.
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Fig11: OpenMalaria function modelling the effect of age on number of mosquito bites received. Shown is the ratio of bites received to those received by an adult by age a(i, t). The continuous line corresponds to proportionality between bites received and expected body surface area. The dotted line shows a similar model which assumes proportionality between bites received and body weight.

Mentions: The dynamics of age-shifts also depend on the slow build-up of natural immunity under parasitological challenge. An important feature of this is an increase in the force of infection with age for the first few years of life [60]. The OpenMalaria models capture this by assuming natural exposure is a function of body size, reflecting increasing biting by mosquitoes on larger hosts [61, 62] Figure 10 (figure taken from [63]). This is implemented as an age-dependent scaling factor, which expresses the number of bites received as a child as a fraction of the bites received by a fully-grown adult, proportional to the ratio in surface areas Figure 11 [63]. The consequent effect of child growth on exposure magnifies the age-shifting effect because when the exogenous protection of the treated cohort is lost at a time when their increased size leaves them more vulnerable than younger children.


Age-shifting in malaria incidence as a result of induced immunological deficit: a simulation study.

Pemberton-Ross P, Smith TA, Hodel EM, Kay K, Penny MA - Malar. J. (2015)

OpenMalaria function modelling the effect of age on number of mosquito bites received. Shown is the ratio of bites received to those received by an adult  by age a(i, t). The continuous line corresponds to proportionality between bites received and expected body surface area. The dotted line shows a similar model which assumes proportionality between bites received and body weight.
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4513612&req=5

Fig11: OpenMalaria function modelling the effect of age on number of mosquito bites received. Shown is the ratio of bites received to those received by an adult by age a(i, t). The continuous line corresponds to proportionality between bites received and expected body surface area. The dotted line shows a similar model which assumes proportionality between bites received and body weight.
Mentions: The dynamics of age-shifts also depend on the slow build-up of natural immunity under parasitological challenge. An important feature of this is an increase in the force of infection with age for the first few years of life [60]. The OpenMalaria models capture this by assuming natural exposure is a function of body size, reflecting increasing biting by mosquitoes on larger hosts [61, 62] Figure 10 (figure taken from [63]). This is implemented as an age-dependent scaling factor, which expresses the number of bites received as a child as a fraction of the bites received by a fully-grown adult, proportional to the ratio in surface areas Figure 11 [63]. The consequent effect of child growth on exposure magnifies the age-shifting effect because when the exogenous protection of the treated cohort is lost at a time when their increased size leaves them more vulnerable than younger children.

Bottom Line: Predicted shifting of burden continue into the second decade of the programme.Conversely, short-lived transient changes in incidence measured soon after introduction of a new intervention may give over-positive views of future impacts.Complementary intervention strategies could be designed to specifically protect those age-groups at risk from burden shift.

View Article: PubMed Central - PubMed

Affiliation: Swiss Tropical and Public Health Institute, 4002, Basel, Switzerland. peter.pemberton-ross@unibas.ch.

ABSTRACT
Effective population-level interventions against Plasmodium falciparum malaria lead to age-shifts, delayed morbidity or rebounds in morbidity and mortality whenever they are deployed in ways that do not permanently interrupt transmission. When long-term intervention programmes target specific age-groups of human hosts, the age-specific morbidity rates ultimately adjust to new steady-states, but it is very difficult to study these rates and the temporal dynamics leading up to them empirically because the changes occur over very long time periods. This study investigates the age and magnitude of age- and time- shifting of incidence induced by either pre-erythrocytic vaccination (PEV) programmes or seasonal malaria chemo-prevention (SMC), using an ensemble of individual-based stochastic simulation models of P. falciparum dynamics. The models made various assumptions about immunity decay, transmission heterogeneity and were parameterized with data on both age-specific infection and disease incidence at different levels of exposure, on the durations of different stages of the parasite life-cycle and on human demography. Effects of transmission intensity, and of levels of access to malaria treatment were considered. While both PEV and SMC programmes are predicted to have overall strongly positive health effects, a shift of morbidity into older children is predicted to be induced by either programme if transmission levels remain static and not reduced by other interventions. Predicted shifting of burden continue into the second decade of the programme. Even if long-term surveillance is maintained it will be difficult to avoid mis-attribution of such long-term changes in age-specific morbidity patterns to other factors. Conversely, short-lived transient changes in incidence measured soon after introduction of a new intervention may give over-positive views of future impacts. Complementary intervention strategies could be designed to specifically protect those age-groups at risk from burden shift.

No MeSH data available.


Related in: MedlinePlus