Given a target of 80% ITN access, the recommended quantification approaches for each scenario varied considerably across countries. Adjustments in quantification for ANC-EPI distribution did not lead to large differences in ITN access in Scenario 1. The key factors driving variation across countries within a given scenario were the estimated retention times for each country and the mean household size. Recommended quantification approaches are summarized for the scenarios that include continuous distribution in Table 2, for 3-year mass campaigns in Table 3, and for 2-year mass campaigns in Table 4.
For Scenario 2, which relies on full-scale annual continuous distribution in combination with routine ANC/EPI ITN delivery to maintain access, the annual quantifier needed to maintain ITN access at 70% ranged from 7% of the population in Eritrea (estimated net retention time of 3.01 years), to 29% of the population in Djibouti, South Sudan, and Chad (retention times of 1.05, 1.02, and 1.03 years, respectively). Similarly, to maintain ITN access at 80%, the quantifier ranged from 10% of the population in Eritrea, to 42% of the population in Mauritania. In only a few countries was ITN access able to reach 90% - requiring a quantification factor of 14% of the population in Equatorial Guinea (retention time of 3.59 years), to 46% of the population in Liberia and Mauritania (1.03 and 1.07 years retention time, respectively).
For Scenario 3, where mass campaigns are conducted every three years, routine distribution of ITNs through ANC/EPI is conducted consistently, and continuous distribution supplements ITN access in the years between campaigns, there was also a range of quantifiers for the annual continuous distribution channels. At the 70% target, ten countries required ITNs equivalent to 0% of the population, essentially indicating that 3-year campaigns with ANC and EPI distribution would maintain ITN access at 70%, but for South Sudan, continuous distribution quantified using population x 28% was needed to maintain ITN access between campaigns. At the 80% target, four countries still achieved this with no continuous distribution ITNs between campaigns, while Liberia, South Sudan, and Chad were estimated to need 35% of the population in ITNs. Only Equatorial Guinea, with an estimated net retention time of 3.59 years, was able to maintain ITN access at 90% quantified with only 8% of the population.
Table 2
Recommended annual quantifiers for continuous distribution channels. All scenarios assume that ANC and EPI delivery of ITNs is ongoing in addition and provides nets to 6% of the population (not shown in table).
Minimum quantifier (population x quantifier, annually) to sustain ITN access at or above specified target level
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Scenario 2 (full continuous distribution strategy)
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Scenario 3 (continuous distribution between mass campaigns)
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Targeted ITN access level:
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Country Code
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Retention time (years)
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70%
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80%
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90%
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70%
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80%
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90%
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LBR
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1.0
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28%
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36%
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46%
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27%
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35%
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GIN
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1.5
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20%
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30%
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38%
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11%
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16%
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31%
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TZA
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2.1
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14%
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20%
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29%
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4%
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11%
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20%
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TGO
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2.4
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12%
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20%
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25%
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1%
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8%
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16%
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COG
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2.9
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10%
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15%
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21%
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0%
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4%
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15%
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CMR
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3.5
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8%
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11%
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15%
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0%
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1%
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9%
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For Scenario 4, the quantifier used for 3-year mass campaigns (in combination with routine ITN distribution at ANC/EPI clinics) was varied from 0.1 to 2.0. The lowest level of ITN access between campaigns is shown in Table 3. Under the current recommended quantifier of 1.8, only Eritrea and Equatorial Guinea, both with net retention times of over three years, were estimated to maintain ITN access at or above 80% between campaigns. Even at population divided by 1.6, only five countries (12.5% of countries in the sample) maintained ITN access above 80%, likewise all with net retention times of at least three years. In Guinea, with the standard quantifier of population / 1.8 and an estimated retention time of 1.51, the country reaches a low point of 46% ITN access between campaigns. Using population / 1.1 would result in an estimated low point of 60% population ITN access between campaigns.
Table 3: Lowest level of ITN access between 3-year campaigns at different population quantifiers. Routine ITN delivery to pregnant women and infants is assumed.
Lowest ITN access between 3-year campaigns for each quantifier
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Table 4 provides a similar picture but for campaigns conducted every 2 years. Under a population / 1.8 quantifier, eighteen countries (45%) including Central African Republic, Rep. Congo, Cameroon, Eritrea, Gabon, Gambia, Guinea, Equatorial Guinea, Kenya, Comoros, Mali, Niger, Nigeria, Sudan, Somalia, Togo, Tanzania, and Zimbabwe would all maintain ITN access at or above 80% between campaigns. In other countries, 2-yearly campaigns closer to one ITN per person would be needed in order to maintain ITN access at the 80% target. Continuing with the Guinea example, with the standard quantifier of population / 1.8, the country reaches a low point of 81% ITN access in the year between campaigns. Using population / 1.2 would result in an estimated low point of 89% population ITN access between campaigns.
Table 4: Lowest level of ITN access between 2-year campaigns at different population quantifiers. Values of 100 indicate excess nets in the system (for example Cameroon). Routine ITN delivery to pregnant women and infants is assumed.
Lowest ITN access between 2-year campaigns for each quantifier
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The below figures illustrate scenario results for each country, for scenario 1 (Fig. 2), scenario 2 (Fig. 3), and scenario 3 (Fig. 4), with predicted population ITN access estimates shown in green and target levels of ITN access highlighted in red at 80% and 90%. The typical rise and fall of ITN access is apparent in Fig. 2 and Fig. 4, while ITN access is maintained at a steady rate in Fig. 3 where distributions are annual through continuous channels.
The complete set of graphs for all 132 scenarios is included as Supplemental File 1.
Table 5 summarizes the recommended quantifiers under each scenario, given a target of maintaining 80% population ITN access. For Liberia, with a median retention time of 1 year, a full scale continuous distribution strategy would need to have an annual ITN need of population x 36%, and a strategy implementing mass campaigns every three years using population/1.8 and adding continuous distribution between campaigns would need to quantify the continuous distribution channel in each non-campaign year using population x 35%. Both strategies assume that ITN delivery to pregnant women and infants is ongoing, reaching about 6% of the population. Under tailored mass campaign scenarios, the models suggest that when net lifetimes are around 12 months, no three-year campaign strategy can achieve universal access; a strategy of campaigns every two years would only maintain ITN access above 80% if the quantification approach is to multiply the population by 0.9 - delivering, in essence, one net per person every other year. In contrast, countries with longer retention times such as Republic of Congo or Cameroon are able to sustain 80% ITN access with full scale continuous strategies that quantify the annual ITN need using population x 15% and 11%, respectively, and could implement continuous distribution between campaigns to sustain access with only population x 4% and 1%, respectively. Alternately, they could implement three-yearly campaigns with no continuous distribution, using population divided by 1.5 and 1.7, along with ITN delivery to pregnant women and infants.
Table 5
Summary of recommended quantifiers for scenarios, to maintain ITN access at or above 80%. The quantifiers are for only the continuous distribution channel or mass campaign; annual ANC/EPI distribution equivalent to 6% of the population is assumed in each scenario, but is not part of the listed quantifiers
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Continuous Distribution
ITNs = Population x X, annually
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Mass Campaign
ITNs = Population / X
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Country
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Retention time (years)
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Full-scale continuous + routine
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Campaign + routine + continuous between campaigns
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3-yearly campaigns
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2-yearly campaigns
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LBR
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1.0
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36%
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35%
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0.9
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GIN
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1.5
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30%
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16%
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0.6
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1.8
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TZA
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2.1
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20%
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11%
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1.0
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2.0
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TGO
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2.4
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20%
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8%
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1.2
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2.0
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COG
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2.9
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15%
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4%
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1.5
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2.0
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CMR
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3.5
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11%
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1%
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1.7
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2.0
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The total number of nets needed under each recommended scenario in Table 5 to maintain 80% ITN access is presented in Fig. 5A, for each country in the sample, and summarized across countries in Fig. 5B. For comparability, results are presented as a percentage difference compared to a status quo scenario of three-yearly campaigns quantified with population/1.8 plus routine distribution of ITNs to pregnant women and infants through ANC and EPI channels. For 15 of the 40 countries, full scale continuous distribution would maintain ITN access at 80% with fewer nets than are currently being delivered under three-yearly status quo campaigns. Across all countries, full scale continuous distribution was expected to maintain ITN access at 80% with 8% more ITNs than status quo. Strategies deploying continuous distribution between mass campaigns required 18.5% more ITNs than status quo, while mass campaign strategies with tailored quantification factors required nearly two and a half times as many ITNs for three-yearly campaigns, and 66.6% more ITNs for two-yearly campaigns.
To assess feasibility of primary school channels delivering the recommended numbers of nets to households, the proportion of the population currently attending primary school was calculated for each region or province in DHS surveys and compared to the population quantifiers needed to achieve 70% and 80% ITN access targets in Scenario 2. The proportion of regions/provinces within each country where primary school students attending school met or exceeded the population quantifier are shown in Fig. 6, as an indication of the extent within a country where annual school distribution would be feasible. This assumes that only one ITN is given per pupil; for the countries in Fig. 6 with a limited proportion of regions where the primary-school-attending population is large enough, giving more than 1 ITN per pupil could provide a solution. Alternatively, additional community-based channels could be designed to distribute ITNs alongside school-based distribution.
Finally, person-years of protection, expressed as person-years of ITN access, can be compared to total nets delivered to assess value for money of different ITN distribution strategies and within them, different quantification factors. Figure 7 shows the quantification options within each scenario that provided the maximum person-years of protection, against the total nets required, for two countries representing shorter and longer retention times, over a 10-year period. While status quo strategies of 3-yearly campaigns quantified using population/1.8 with ANC/EPI at 7% require the fewest ITNs, they also provide the fewest person-years of ITN access. Moving rightward along the x-axis in Fig. 7A, we find options that provide more person-years of ITN access, each with slightly more total nets required. However, in Fig. 7B, with a nearly three-year retention time, annual CD quantified at population x 15% provides 168 million person-years of ITN access over 10 years, with other options requiring more total nets but providing only minimal, if any, additional protection. Moreover, countries with shorter retention times require far more nets overall in order to provide similar levels of protection compared to countries with longer retention times: a full-scale CD program in country A achieves less than 159.5 million person-years of ITN access for around 82.3 million ITNs, compared to country B, where full scale CD provides 168.5 million person-years of ITN access for around 44.2 million ITNs. Frontier plots for all countries are included in Supplemental File 2.