How many mosquito nets are needed to maintain universal coverage: an update

Abstract

Background: Insecticide-treated nets (ITNs) have served as the cornerstone of malaria vector control in sub-Saharan Africa for the past two decades. Over 2.5 billion ITNs have been delivered since 2004 primarily through periodic mass distribution campaigns scheduled at approximately three-year intervals, aligning with the expected lifespan of nets. Recent work indicates that ITN retention times are less than two years in most countries, raising key questions for quantification approaches and delivery frequency for ITN distribution. This paper models several quantification approaches for five typical ITN distribution strategies, estimates the proportion of the population with access to an ITN, and presents recommended quantification approaches to meet global targets for ITN access and use.

Methods: A stock and flow model with annual timesteps was used to model ITN distribution and resulting ITN access for 2020-2035 under five scenarios in 40 countries: (1) three-year mass campaigns, (2) full-scale annual continuous distribution, (3) three-year mass campaigns plus continuous distribution in the years between campaigns, (4) three-year mass campaigns at different quantification approaches, (5) two-year mass campaigns at different quantification approaches. All scenarios included ITN distribution to pregnant women at antenatal clinics and infants at immunization visits.

Results: The current status quo of conducting mass campaigns every three years using a population/1.8 quantifier is insufficient to achieve or maintain targets of 80% population access to ITNs in most malaria-endemic countries, given most estimated retention times are less than three years. Tailored three- or two-year mass campaigns were less efficient than annual continuous distribution strategies in nearly all settings. For countries with at least 2.5 year median ITN retention times, full scale continuous distribution provided better ITN access while needing 20-23% fewer ITNs compared to current mass campaigns.

Conclusion: Given variation in ITN retention times across countries, tailored quantification approaches for mass campaigns and continuous distribution strategies are warranted. Continuous distribution strategies are likely to offer more efficient ways to maintain ITN coverage, with fewer nets, where ITN retention times are at least two and a half years. National malaria programmes and their funding partners should work to increase the number of ITNs available to those vulnerable to malaria, while at the same time working to extend the useful life of these critical commodities.

Fig. 1

A ITN retention times estimated by Malaria Atlas Project (MAP) and median lifespans estimated from durability monitoring (DM) studies; countries are labeled by IS03 code. B Smooth-compact loss function for net decay C Nonparametric conditional quantile function for ITN access as a function of nets-per-capita (NPC)

Fig. 2

Estimated ITN access under a typical three-year mass campaign strategy, with ANC/EPI distribution at 6% of the population annually. Red lines indicate 80% and 90% ITN access targets

Fig. 3

Estimated ITN access with annual ANC/EPI at 6% and full continuous distribution strategy at 17% of the population in nets each year. Shaded areas indicate 95% confidence intervals accounting for both net retention times and ITN access as a function of nets-per-capita (NPC). Red lines indicate 80% and 90% ITN access targets

Fig. 4

Scenario 3—Estimated ITN access under three-year mass campaigns with ANC/EPI distribution at 6%, and between-campaign continuous distribution at 10%. Red lines indicate 80% and 90% ITN access targets

Fig. 5

Relative proportions of total ITNs needed over 10 years, compared to status quo (three-yearly campaigns at population/1.8), by scenario and ITN retention time. Grey labels show group average for each scenario

Fig. 6

Percentage of regions/provinces per country where large-scale annual school distribution to achieve ITN access targets for A 70% and B 80% is feasible, assuming one ITN is given per child

Fig. 7

Frontier plots of person-years of ITN access vs total nets delivered, in illustrative populations of 10 million people for comparability, for A a country with a median retention time of 1.05 years and B a country with a median retention time of 2.91 years. Results for all countries are included in Additional File 3