Net coverage; how much is enough?

We are unlikely to eliminate mosquitoes, according to Tanya Russell and colleagues, but she notes that this should not stop us from implementing all available interventions. Specifically their study of malaria vectors in Tanzania found that the at reduced densities of mosquito populations, they try to reproduce more, meaning we may never get below 10% mosquito elimination.

Instead, a member of the National Malaria Control Program in Tanzania says our goal “should be to reduce, and eventually halt, transmission of the parasite, rather than eliminating the vector.” If we can achieve no more than 90% elimination of mosquitoes, what is a realistic coverage figure for malaria interventions?

Applications of net and case management strategies in Rwanda and Ethiopia have definitely shown that major drops in malaria incidence are possible.  But the RBM targets of 80% coverage (85% for the US President’s Malaria Initiative) are elusive.  Demographic and malaria surveys from Senegal, Liberia and Nigeria show that even in homes that own nets, net use among people at most risk, does not reach this target.

Are we really sure that 80% is the right target?

Fred Binka was one of the first to demonstrate that people living in homes without nets can be protected by their neighbors’ nets, which kill mosquitoes in the community. ITNs “provided very good personal protection to children using them, and also protected nonusers in nearby compounds. Among nonusers, the mortality risk increased by 6.7% with each additional shift of 100 m away from the nearest compound” with nets. This led the researchers to speculate on the need to study whether the “mass effect from a small number of highly dispersed nets would provide equivalent protection to complete coverage.”

A few years later William Hawley and co-researchers reported that, “protective effect of ITNs on compounds lacking ITNs located within 300 meters of compounds with ITNs for child mortality, moderate anemia, high-density parasitemia, and hemoglobin levels.”

As part of the move toward universal coverage, Killeen and colleagues examined the importance of considering all household members, not just the ‘vulnerable.’ The group condluded that …

Using field-parameterized malaria transmission models, we show that high (80% use) but exclusively targeted coverage of young children and pregnant women (20% of the population) will deliver limited protection and equity for these vulnerable groups. In contrast, relatively modest coverage (35%–65% use, with this threshold depending on ecological scenario and net quality) of all adults and children, rather than just vulnerable groups, can achieve equitable community-wide benefits equivalent to or greater than personal protection.

Barat has called for ‘data driven decision making‘ in the effort to eliminate malaria. Using data in models as done by Killeen is a further important step. The onchocerciasis control community has been working with such models for over 15 years now. New data are fed into the Onchosim model based on program progress such that it is possible to forecast that onchocerciasis could be eliminated from areas with high initial prevalence if 65% coverage of ivermectin treatment were maintained for at least 25 years.

Unlike onchocerciasis control, malaria elimination rests on multiple interventions.  This makes modeling much more urgent, as outlined by malERA’s research agenda for eradication. Since universal coverage unfortunately does not mean universal usage, we need to seek valid data and models to help us plan for distribution of malaria interventions more strategically in ways that are affordable and can be maintained and at the same time can achieve maximum reductions in morbidity and mortality.

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