Modeling resource allocation strategies for insecticide-treated bed nets to achieve malaria eradication

  1. Nora Schmit  Is a corresponding author
  2. Hillary M Topazian
  3. Matteo Pianella
  4. Giovanni D Charles
  5. Peter Winskill
  6. Michael T White
  7. Katharina Hauck
  8. Azra C Ghani
  1. MRC Centre for Global Infectious Disease Analysis, Imperial College London, United Kingdom
  2. Infectious Disease Epidemiology and Analytics G5 Unit, Department of Global Health, Institut Pasteur, Université de Paris, France
  3. MRC Centre for Global Infectious Disease Analysis, Jameel Institute, Imperial College London, United Kingdom
17 figures, 6 tables and 1 additional file

Figures

Global distribution of P. falciparum and P. vivax malaria burden in 2000 (in the absence of insecticide-treated nets) obtained from the Malaria Atlas Project (Weiss et al., 2019; Battle et al., 2019).

(A) The annual number of clinical cases and (B) the population at risk of malaria across settings with different transmission intensities and endemic for P. falciparum, P. vivax, or co-endemic for both species. The number of countries in each setting is indicated below the figure.

Modeled impact of insecticide-treated net (ITN) usage on malaria epidemiology by the setting-specific transmission intensity, represented by the baseline entomological inoculation rate.

The impact on the clinical incidence and prevalence of P. falciparum malaria (panels A and B) and on the clinical incidence and prevalence of P. vivax malaria (panels C and D) is shown. Panels A and C represent the clinical incidence for all ages.

Global clinical cases and population at risk of malaria under different allocation strategies at varying budgets.

The impact on total malaria cases (panel A), total population at risk (panel B), individual P. falciparum and P. vivax cases (panel C), and population at risk of either species (panel D) are shown. Budget levels range from 0, representing no usage of insecticide-treated nets, to the budget required to achieve the maximum possible impact. Optimizing for case reduction generally leads to declining populations at risk as the budget increases, but this is not guaranteed due to the possibility of redistribution of funding between settings to minimize cases. The strategy optimizing case reduction and pre-elimination shown here places the same weighting (1:1) on reaching pre-elimination in a setting as on averting total cases, but conclusions were the same for weights of 0.5–100 on pre-elimination.

Optimal strategy for funding allocation across settings to minimize malaria case burden at varying budgets.

Panels show optimized allocation patterns across settings of different transmission intensities (panels A and B) and different endemic parasite species (panels C and D). The proportion of the total budget allocated to each setting (panels A and C) and the resulting mean population usage of insecticide-treated nets (ITNs) (panels B and D) are shown.

Appendix 1—figure 1
Malaria transmission model; diagram adapted from Griffin et al., 2016 and White et al., 2018.

Humans move through six states in the models for both species: S (susceptible), D (untreated symptomatic infection), T (successfully treated symptomatic infection), A (asymptomatic infection), U (asymptomatic sub-patent infection), and P (prophylaxis) in the P. falciparum model, and S (susceptible), ID (untreated symptomatic infection), T (successfully treated symptomatic infection), ILM (asymptomatic light microscopy detectable blood-stage infection), IPCR (asymptomatic sub-microscopic PCR detectable blood-stage infection) and P (prophylaxis) in the P. vivax model. New infections (including superinfections) are highlighted in blue but parameters are not shown. Rates rD, rA, rU, rT, rP, rLM, and rPCR determine the mean duration of each state. Hypnozoites states in the P. vivax model are not shown on the diagram. Adult female mosquitoes move through three model compartments: Sm (susceptible), Em (exposed), and Im (infected).

Appendix 1—figure 2
Modeled impact of insecticide-treated net (ITN) usage on clinical incidence over time for three representative entomological inoculation rates (EIR) for (A) P. falciparum and (B) P. vivax.

EIRs represent the minimum, median, and maximum EIRs of the global population distribution. Lines represent increments of 1% of ITN usage.

Appendix 1—figure 3
Relationship between access and nets per capita in 2020 (generated from data in Bertozzi-Villa et al., 2021).
Appendix 1—figure 4
Surface plots of entomological inoculation rate (EIR) vs. insecticide-treated net (ITN) usage vs. clinical incidence.

Plots were fit using bivariate linear interpolation of gridded data EIR and ITN usage values taken from mathematical model simulations.

Appendix 1—figure 5
Prevalence of P. falciparum in children 2–10 years (2000), matched to entomological inoculation rate (EIR) values by country.

Points are sized by total population and colored by World Health Organization region.

Appendix 1—figure 6
Prevalence of P. vivax in people 0–99 years (2000), matched to entomological inoculation rate (EIR) values by country.

Points are sized by total population and colored by World Health Organization region.

Appendix 1—figure 7
Percentage of settings not having reached pre-elimination (<1 case per 1000 population at risk) under different allocation strategies at varying budgets.

Budget levels range from 0, representing no usage of insecticide-treated nets, to the budget required to achieve the maximum possible impact. For the strategies optimizing for case reduction and pre-elimination, brackets show the weight placed on averting total cases vs on reaching pre-elimination in the optimization (i.e. 1:1 represents equal weight on both).

Appendix 1—figure 8
Illustration of resource allocation patterns under the three modeled policy strategies at varying budgets.

(A) Percentage of global funding allocated to each transmission setting. (B) Mean funded insecticide-treated net (ITN) usage in each transmission setting. Transmission intensity groups represent transmission settings with varying population sizes proportional to the global distribution.

Appendix 1—figure 9
Optimal strategies for funding allocation across settings to minimize malaria case burden (top panels) and to minimize malaria cases and increase the number of settings reaching a pre-elimination phase at varying budgets.

Panels show the proportion of the budget allocated and the resulting mean population usage of insecticide-treated nets (ITNs) across settings of different transmission intensities. For the strategies optimizing for case reduction and pre-elimination, brackets show the weight placed on averting total cases vs on reaching pre-elimination in the optimization (i.e. 1:1 represents equal weight on both).

Appendix 1—figure 10
Influence of different assumptions about the relationship between the cost and population usage of insecticide-treated nets (ITNs) on the impact of the allocation strategies.

The global clinical malaria cases (panel A) and the population at risk of malaria (panel B) under different allocation strategies are shown at varying budgets. Results with the more realistic non-linear assumption are presented throughout the main manuscript. Budget levels are expressed relative to the maximum budget required to achieve the largest possible impact with ITNs, but note that this maximum budget was different depending on the ITN costing assumption ($3,698,727,241 for non-linear vs. $6,902,923,309 for linear).

Appendix 1—figure 11
Resource allocation patterns over time for P. falciparum.

Panel A shows the number of cases over time for re-allocation of insecticide-treated nets (ITNs) every 3 years compared to the one-time allocation of a constant ITN usage to minimize the final (year 39) case burden. Panels B and C show the optimal allocation pattern for each 3 year distribution cycle across settings of different transmission intensities. The maximum budget was 26.6 million for ITN distributions every 3 years over 39 years.

Appendix 1—figure 12
Impact of the proportional allocation strategy and the 2020–2022 Global Fund allocation on global malaria cases (panel A) and the total population at risk of malaria (panel B) at varying budgets.

Both strategies use the same algorithm for budget share allocation based on the malaria disease burden in 2000–2004, but the Global Fund allocation additionally involves an economic capacity component and specific strategic priorities (The Global Fund, 2023).

Author response image 1
Impact of the proportional allocation strategy and the 2020-2022 Global Fund allocation on global malaria cases (panel A) and the total population at risk of malaria (panel B) at varying budgets.

Both strategies use the same algorithm for budget share allocation based on malaria disease burden in 2000-2004, but the Global Fund allocation additionally involves an economic capacity component and specific strategic priorities.

Tables

Table 1
Relative reduction in malaria cases and population at risk under different allocation strategies.

Reductions are shown relative to the baseline of 321 million clinical cases and 4.1 billion persons at risk in the absence of interventions. Low, intermediate, and high budget levels represent 25%, 50%, and 75% of the maximum budget, respectively. The strategy optimizing case reduction and pre-elimination shown here places the same weighting (1:1) on reaching pre-elimination in a setting as on averting total cases.

Clinical casesPopulation at risk
ScenarioBudget levelNumber (millions)Relative reduction (%)Number (billions)Relative reduction (%)
Optimized for case reductionLow136.1584.10
Intermediate77.0763.96
High53.9832.442
Maximum41.5870.491
Optimized for case reduction & pre-eliminationLow161.3504.03
Intermediate87.8733.516
High58.8821.858
Maximum41.5870.491
Prioritize high-transmission settingsLow153.9524.02
Intermediate109.5663.417
High61.8813.319
Maximum41.5870.491
Proportional allocationLow166.9484.11
Intermediate150.4534.04
High123.8614.04
Maximum116.0643.95
Prioritize low-transmission settingsLow268.2172.344
Intermediate245.2241.856
High202.1370.588
Maximum41.5870.491
Table 2
Overview of modeled scenarios for allocation of funding to different transmission settings.

Strategies 1A-1E compare resource allocation scenarios using clinical incidence values from each transmission setting at equilibrium after insecticide-treated net (ITN) coverage has been introduced. Strategies 2A-2B are compared as part of the allocation over time sub-analysis. EIR: entomological inoculation rate.

StrategyModeling approach/assumptions
1AOptimized for total malaria case reductionGeneralized simulated annealing is used to determine the optimal allocation of a given budget to minimize the total number of global malaria cases.
1BOptimized for total malaria case reduction and pre-eliminationGeneralized simulated annealing is used to determine the optimal allocation of a given budget to minimize the total number of global malaria cases while placing a premium on the pre-elimination phase being reached in a setting.
1CPrioritize high-transmission settingsFunding is allocated to groups of countries according to transmission intensity (P. falciparum + P. vivax entomological inoculation rate, EIR). For a given budget, the transmission settings with the highest EIR are prioritized, increasing ITN coverage in increments of 1% in each setting until malaria is eliminated or until an increase in coverage leads to no further decrease in cases, before allocating to the next-highest EIR setting.
1DPrioritize low-transmission (near-elimination) settingsFunding is allocated to groups of countries according to transmission intensity (P. falciparum + P. vivax EIR). For a given budget, the transmission settings with the lowest EIR are prioritized, increasing ITN coverage in increments of 1% in each setting until malaria is eliminated or until an increase in coverage leads to no further decrease in cases, before allocating to the next-lowest EIR setting.
1EProportional allocationFunding is allocated to groups of countries in proportion to their disease burden. Budget shares are calculated using country data from the World Malaria Report (World Health Organization, 2007; World Health Organization, 2020) and account for the country-specific total malaria cases (P. falciparum and P. vivax), deaths, incidence and mortality rate in 2000–2004, scaled by the subsequent increase in the population at risk (The Global Fund, 2019).
2AOne-time optimized allocation for P. falciparum case reductionGeneralized simulated annealing is used to determine the optimized allocation at a given budget, minimizing the total number of global P. falciparum cases after 39 years, resulting in constant ITN usage in each setting over this time period.
2BOptimized allocation every three years for P. falciparum case reductionGeneralized simulated annealing is used to determine the optimized allocation at a given budget, minimizing the total number of global P. falciparum cases after every 3 year period for 39 years, allowing ITN usage to vary in each setting every 3 years.
Appendix 1—table 1
P. falciparum human model parameter values.

Full details can be found in the original publication (Griffin et al., 2016), including references for parameters and intervals for the prior and posterior distributions (median values of the posterior distribution are used in model simulations).

ParameterSymbolEstimate
Human infection duration (days)
Latent perioddE12
Patent infection1rA195
Clinical disease (untreated)1rD5
Treatment of clinical disease1rT5
Sub-patent infection1rU110.299
Prophylaxis1rP15
Age and heterogeneity
Age-dependent biting parameterρ0.85
Age-dependent biting parametera08 years
Variance of the log heterogeneity in biting ratesσ21.67
Pre-erythrocytic immunity reducing probability of infection
Duration of refractory period in which immunity is not boosteduB7.19919 days
Duration of pre-erythrocytic immunitydB10 years
Maximum probability of infection due to no immunityb00.590076
Maximum relative reduction in probability of infection due to immunityb10.5
Scale parameterIB043.8787
Shape parameterKB2.15506
Immunity reducing probability of clinical disease
Duration of refractory period in which immunity is not boosteduC6.06349 days
Duration of clinical immunitydCA30 years
New-born immunity relative to mother’s clinical immunityPCM0.774368
Duration of maternal immunitydCM67.6952 days
Maximum probability of clinical disease due to no immunityΦ00.791666
Maximum relative reduction in probability of clinical disease due to immunityΦ10.000737
Scale parameterIC018.02366
Shape parameterKC2.36949
Immunity reducing probability of detection
Duration of refractory period in which immunity is not boosteduD9.44512 days
Duration of detection immunitydID10 years
Minimum probability of detection due to maximum immunityd10.160527
Scale parameterID01.577533
Shape parameterKD0.476614
Scale parameter relating age to immunityaD21.9 years
Time-scale at which immunity changes with agefD00.007055
Shape parameter relating age to immunityγD4.8183
Appendix 1—table 2
P. vivax human model parameter values.

Full details can be found in the original publication (White et al., 2018) including references for parameters and intervals for the prior and posterior distributions.

ParameterSymbolEstimate
Human infection duration (days)
Latent perioddE10
Light microscopy-detectable asymptomatic infection1rLM10
Clinical disease (untreated)1rD5
Treatment of clinical disease1rT1
Prophylaxis1rP28
Age, heterogeneity, and probability of infection
Age-dependent biting parameterρ0.85
Age-dependent biting parametera08 years
Variance of the log heterogeneity in biting ratesσ21.29
Probability of blood-stage infection upon infectious mosquito biteb0.5
Hypnozoite parameters
Relapse ratef0.024 per day
Clearance rateγL0.0026 per day
Maternal immunity
New-born immunity relative to mother’s clinical immunityPmat0.421
Duration of maternal immunitydmat35.148 days
Anti-parasite immunity reducing probability of light microscopy-detectable infection and duration of PCR-detectable infection
Duration of refractory period in which immunity is not boostedupar19.77 days
Duration of anti-parasite immunity1rpar10 years
Maximum probability of detectability by light microscopy due to no immunityΦLM,max0.8918
Minimum probability of detectability by light microscopy due to full immunityΦLM,min0.0043
Scale parameter for detectability by light microscopyALM,50%27.52
Shape parameter for detectability by light microscopyKLM2.403
Maximum duration of PCR-detectable infection due to no immunitydPCR,max70 days
Minimum duration of PCR-detectable infection due to full immunitydPCR,min10 days
Scale parameter for duration of PCR-detectable infectionAPCR,50%9.9
Shape parameter for duration of PCR-detectable infectionKPCR4.602
Clinical immunity reducing probability of clinical disease
Duration of refractory period in which immunity is not boosteduC7.85 days
Duration of detection immunity1rC30 years
Maximum probability of clinical disease due to no immunityΦD,max0.8605
Minimum probability of clinical disease due to full immunityΦD,min0.018
Scale parameter for clinical diseaseAD,50%11.538
Shape parameter for clinical diseaseKD2.250
Appendix 1—table 3
Mosquito model and insecticide-treated net (ITN) parameter.

Full details on parameter values can be found in the original publications (Griffin et al., 2010; Griffin et al., 2016; White et al., 2011; White et al., 2018), including references and intervals for the prior and posterior distributions for fitted parameters (median values of the posterior distribution are used in model simulations).

P. falciparum (Anopheles gambiae s.s.)P. vivax (Anopheles punctulatus)
Infectiousness of humans to mosquitoes
Lag from parasites to infectious gametocytesτ112.5 days-
Untreated clinical diseasecD0.0680.8
Treated clinical diseasecT0.0220.4
Sub-patent infectioncU0.0062-
Parameter for infectiousness of asymptomatic infectionγ11.82425-
Light microscopy-detectable infectioncLM-0.1
PCR-detectable infectioncPCR-0.035
Mosquito population model
Daily mortality of adult mosquitoes with no interventionsμ00.1320.167
Extrinsic incubation periodτM10 days8.4 days
Larval model
Early instar larval developmental perioddE6.64 days6.64 days
Late instar developmental perioddL3.72 days3.72 days
Pupal developmental perioddP0.643 days0.643 days
Daily mortality rate of early-stage larvae (density-dependent)μE0.03380.0338
Daily mortality rate of late-stage larvae (density-dependent)μL0.03480.0348
Daily mortality rate of pupae (density-independent)μP0.2490.249
Effect of density dependence on late instars relative to early instarsγ13.2513.25
Maximum number of eggs per oviposition per mosquitoβLmax21.221.2
Mosquito behavior
Mean duration of host-seeking in the absence of vector control interventionsδ10.69 days0.69 days
Mean duration of resting between blood mealsδ22.31 days2.31 days
Proportion of bites taken on humans (anthropophagy) in the absence of vector control interventionsQ00.920.5
Proportion of bites taken on humans indoors and in bedΦb0.890.9
Effect of ITNs
Maximum probability of a mosquito being repelled by a ITN with full insecticidal and barrier effectrN00.560.6
Minimum probability of a mosquito being repelled by a ITN after decayrNM0.240.2
ITN half-life-2.64 years2.64 years
Maximum probability of a mosquito being killed by a ITN with full insecticidal and barrier effectdN00.410.3
Appendix 1—table 4
Parameter values for the insecticide-treated net (ITN) costing conversion.
ParameterSymbolValueSource
ITN usage-Varies in simulations-
ITN use rate (proportion)-0.84Median across African countries in 2019 (Bertozzi-Villa et al., 2021)
ITN half-life (years)-1.64Median across African countries in 2020 (Bertozzi-Villa et al., 2021)
ITN distribution frequency (years)DF3World Malaria Report (World Health Organization, 2007; World Health Organization, 2020)
Net loss function rate parameterκ20Bertozzi-Villa et al., 2021

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  1. Nora Schmit
  2. Hillary M Topazian
  3. Matteo Pianella
  4. Giovanni D Charles
  5. Peter Winskill
  6. Michael T White
  7. Katharina Hauck
  8. Azra C Ghani
(2024)
Modeling resource allocation strategies for insecticide-treated bed nets to achieve malaria eradication
eLife 12:RP88283.
https://doi.org/10.7554/eLife.88283.3