Currently, targets in national strategic plans or donor documents for all three ITN coverage indicators are usually set at 80% or above. This gives a perhaps unintended implication that the three indicators should increase together at the same rates. Moreover, universal coverage guidance from WHO and others calls for procuring ITNs with the goal of providing each household with 1 ITN for 2 people, again implying that by doing so, countries should expect to achieve 100% of households owning enough ITNs immediately after a mass campaign. These implied expectations contribute to confusion and frustration when post-campaign results—particularly for the indicator of households owning at least 1 ITN for 2 people—are far below target levels. This also has significant implications for donor funding. Performance frameworks, typically aligned to the household ownership indicators, may inadvertently set national programmes up for failure, and countries may be unnecessarily penalized based on the indicator targets, not all of which are achievable.
This work builds on previous work describing the utility of the ITN indicators and exploring the mathematical relationships between the indicators in order to model ITN coverage in years between surveys [6, 9, 11]. However, in terms of programmatic utility, each indicator must be considered with its advantages and limitations, in terms of interpreting the extent of ITN protection in a given population. The two household indicators can be compared to provide an ‘ownership gap’, and the two population indicators can be compared to provide a ‘use gap’ [9, 11].
The proportion of households owning at least 1 ITN is a minimal threshold that essentially describes the spatial reach of ITN distribution activities, but not the degree to which the population is protected. (The vast majority (80%) of households in endemic countries require more than a single ITN to protect all persons in the household—see Additional file 2). At the other end of the spectrum, the proportion of households owning at least 1 ITN for 2 people is an indicator of ‘perfect’ household coverage, and has never been reached at a national or even a subregional level. Households may miss qualifying as having ‘enough’ nets by only 1–2 nets, and this is often misinterpreted as these households not having any protection. The two household-level indicators—one representing minimal coverage, the other only ‘universal’ coverage—thus provide an incomplete and potentially misleading picture of personal protection and the success of an ITN distribution programme.
Larger households were far less likely than smaller households to own enough ITNs for all their household members. In fact, many individuals in these households that own some but not enough ITNs had access to a net, and (in most cases) were sleeping under one, as illustrated in Fig. 6. This individual protection is obscured when programme planners focus only on the household-level indicator.
Given these limitations of the household level indicators, the population ITN access indicator is a far better indicator of ‘universal coverage’ because it is based on individual people. It provides a clear picture of the proportion of individuals in a given setting that have the opportunity to use an ITN. It can also be directly compared to the proportion of the population that used an ITN the previous night, which enables detailed analysis of specific behavioral gaps nationally as well as among population subgroups. Ultimately, of course, ITN use is the key behavior required for malaria control, but people cannot use an ITN to which they do not have access. Recent research demonstrates clearly that rates of ITN use among those with access to an ITN with few exceptions are at or above an 80% target [10,11,12,13]. Therefore, increasing ITN access will lead directly to increases in ITN use.
It is important to consider and address the programmatic and policy factors that prevent households from obtaining enough ITNs. The primary programmatic reason is that larger households rarely receive the necessary number of ITNs during mass campaigns. During the process of household registration, programmes often put a cap on the number of ITNs any given household can receive to reduce the opportunity for fraud due to inflated numbers of household members. Second, during the process of distribution itself, campaign staff may also pragmatically ration ITNs if they think not enough are available, to remain certain that all households in their catchment area will receive at least some ITNs. Third, larger households, which tend to have more children than smaller households, may have some sleeping spaces in which more than 2 people are sharing an ITN  and, therefore, may not require (or be motivated to acquire) ITNs in the 1:2 ratio.
Policy decisions are also likely affecting the ability of large households to receive enough ITNs. The current guidance from the WHO recommends that mass campaigns be planned with a quantification algorithm of “population divided by 1.8,” which is intended to account for the odd-numbered households that require an additional ‘half net’ according to the 1 for 2 ratio. In 2017 the algorithm was updated by the WHO to allow a 10% buffer accounting for outdated census projections . However, when looking more closely at sleeping space patterns, researchers have found reasons to question the current quantification guidance. Analysis of discordant ITN-person pairs (e.g., two roommates in a household who do not share a sleeping space) as well as the trend that with increasing wealth and also increasing ITN access, the number of people per ITN decreases, imply that a more accurate expectation would be that each ITN only protects on average 1.6 people . Whether inaccurate population estimates or an inadequate quantification factor are the greater determinant of ITN gaps during mass campaigns remains to be explored.
Three possible solutions to these supply challenges can be considered. First, if the current levels of ITN access are considered sufficient in epidemiological terms to maintain malaria control, then no changes in distribution strategy are required. Rather, the current targets could be adjusted to be more appropriate for each indicator. Enormous reductions in malaria morbidity and mortality have been observed over the past decade, during which ITN coverage has not been at target levels and yet this ‘insufficient’ ITN coverage has been credited with two-thirds of the observed reductions in morbidity . Modeling work also indicates that community-level protection can be achieved at lower-than-universal coverage levels of 35–65% population use of ITNs . On this basis, the target levels for each ITN coverage indicator might be adjusted to be more pragmatic—i.e. a 95% target for household ownership of any ITN, which corresponds roughly to an 80% target for population ITN access, 70% population ITN use, and a 55% target for household ownership of at least 1 ITN for 2 people.
Second, if current levels of ITN access are not considered sufficient for malaria control, and targets for ITN use should be 80%, as outlined in the WHO Global Technical Strategy (implying a population ITN access target of 90%) , then it follows that additional ITNs would need to be procured, potentially using a 10% buffer for mass campaigns, and/or by increasing ITN distribution through ongoing school or community based channels. The WHO currently calls for mass campaigns every 3 years using the population/1.8 quantification factor with an optional 10% buffer, which is equivalent to a population/1.6 quantification. The guidelines further recommend additional ITN distribution as needed to maintain target levels , but there is no robust guidance on how many additional ITNs might be needed, nor the most efficient combination of distribution strategies, which inhibits programmes from moving away from triennial mass campaigns. Additional research to provide more specific estimates, including cost-effective options for optimizing ITN coverage over time and space, would likely ease this process for programme planners and donors.
Third, there is some scope for increasing programmatic efficiency in ITN distribution and attempting to use existing quantities of ITNs to achieve higher rates of coverage. This can be done regardless of whether the above two solutions are implemented. First, programmes must acknowledge that large households require more nets, and either avoid setting caps, or set them taking into account regional demographic variations in household sizes. Data on household size are regularly reported in large national surveys such as DHS and MIS, and are summarized here in Additional file 2. Caps currently serve both to limit the negative impact of respondents inflating household size during registration and to ensure that in situations where not enough ITNs are available, all households receive at least a few ITNs. Some regions may indeed have only a small percentage of households that are larger than 8 people; a cap of four ITNs per household might work well. However, in other regions, a cap of four ITNs per household may automatically exclude 15% of households from reaching the target of 1 ITN for 2 people, as in Ghana’s northern regions where the average household size is larger than the rest of the country . Obviously, additional mechanisms to avoid inflation of the numbers of household members during the campaign’s registration phase are and should be put in place. In areas where caps have been used, ‘deflation’ of household size has also been observed—splitting larger households into two or more smaller households to avoid the cap. Second, it has been shown previously that the quality of census and household registration data contribute much more to successful campaigns than other factors . Therefore, investing in household registration and its supervision will help to ensure that all households—whether large, small, or hard to reach—are reached and accurately served. Additional research will be needed to thoroughly assess cost-effective strategies for capping.
There are some minor methodological factors related to achieving universal coverage (based on either indicator) that should be noted. The standard MIS/DHS net roster only lists up to seven ITNs, ignoring any additional nets in the household. In the Mali 2015 MIS, 13% of households owned 7 ITNs, and it is likely that many households own 8 or more nets. These additional nets, however, are not counted and, therefore, these households (if large) may miss reaching the threshold of owning 1 ITN for 2 people solely as a result of this approach. Other countries, including Senegal, have modified the standard net roster to allow for additional ITNs to fit their context. Likewise, the definition of a household—whether for household survey purposes or for mass campaign planning and registration—is certain to be problematic if not done consistently.