Estimating the potential gains and savings from increased
mosquito net durability
Albert Kilian
Malaria Consortium through the NetWorks Project
funded by
Summary
An attempt was made to explore the savings that could be gained from an increased durability
of long-lasting insecticidal nets (LLIN) for the prevention of malaria in sub-Sahara Africa. Using
the NetCALC tool the number of LLIN needed annually for the next 10 years to sustain
universal coverage (assumed to be achieved in 2011) was calculated for net durability varying
from 2 to 7 years with 1 year increases. All comparisons were made relative to the need and
cost of a 3-year LLIN with the following main results:
• The savings in annual LLIN demand range from 22% for a 4-year durability to 50% for
an LLIN that lasts on average 7 years and are expected to be even higher in the first
few years following the scale-up campaign.
• Based on these savings the unit cost for a 7-year LLIN (procurement only or including
distribution, i.e. cost per net delivered) could be twice as high as that for a 3-year LLIN
without increasing the overall cost, i.e. financial savings could be made as long as the
unit cost are less than 2.0 fold. For a 4-year LLIN the equal cost ratio would be still
about 1.3 times the cost for the 3-year LLIN. In addition, the transactional cost would
also be reduced.
• Applying these findings to the sub-Sahara population at risk of malaria (765 million)
shows that the annual need in LLIN to sustain universal coverage would reduce from
130-157 million per year for the 3-year LLIN to 65 to 80 million for a 7-year LLIN.
• Assuming a unit cost of US$ 4.50 for a 3-year LLIN and an increase of US$ 0.50 in unit
cost for each year gained durability, the cumulative savings in sub-Sahara Africa could
reach US$ 439 million until 2015 for a one year increase in durability, i.e. a 4-year LLIN
and US$ 1 billion for a 7-year LLIN
These finding are encouraging as they show the tremendous potential of increasing the
durability of LLIN, but they have to be interpreted with caution as some assumptions used in
the projections are not yet validated by field data. In addition, these savings can only be
realized if the distribution strategies are able to adjust to this longer duration.
1
Introduction
It is increasingly recognized by the international community engaged in malaria control that
understanding and improving the durability of long-lasting insecticidal nets (LLIN) in the field is
critical for the success of malaria prevention using mosquito nets. Some progress has been
made in the development of a standardized methodological approach for the assessment of
the physical integrity of nets in the field with the recently published guideline from WHO [1]. It
is hoped that now data on net survival and durability of different brands in different
environments will become available within the next one to two years. However, irrespective of
field data it can already be said that in principle two mechanisms are available to prolong the
useful life of a mosquito net [2]:
a) by using materials and yarns that provide more resistance against the common
stresses a net is exposed to, i.e. by procuring products that can be expected to last
longer rather than the one with the lowest price and
b) by changing people’s behavior with respect to care and repair of nets, i.e. avoiding
holes and repairing them early to prevent deterioration.
An important question is then, how many fewer LLIN would be needed to sustain universal
coverage as part of the Global Malaria Control Strategy and how much money could be saved
for each year a net could last longer? This question can easily and systematically be addressed
by modeling and some first results are presented here.
Methods
Projections were carried out with NetCALC, a tool to support countries to plan for a
comprehensive distribution strategy for LLIN (publication in preparation). NetCALC assumes
durability of an LLIN product to be the median survival time of the physical nets, i.e.
insecticidal protection is assumed to last for the physical life of the net. The loss function over
time is modeled as an S-shaped decline and can be varied between 2- and 7-year durability
(Figure 1). While for the currently available LLIN products there is increasing data to support
such a loss function [2], the loss dynamics are still hypothetical for nets with longer durability
as no field data exists to date.
Calculations are based on the 2010 population at risk of malaria in sub-Sahara Africa reported
by the World Malaria Report 2010 as 765 million and carried forward applying a 2.34% annual
growth rate (population weighted mean from UNPOP 2010). It was assumed that by the end of
2011 sub-Sahara Africa has reached universal coverage with LLIN, i.e. 100% household
ownership coverage with one LLIN for every two people equivalent to population divided by
the factor 1.8 [3]. For each following year the number of annually needed LLIN was calculated
to sustain a 100% universal coverage for each LLIN durability from a 2-year to a 7-year LLIN.
This data was then exported to another spreadsheet for the cost calculations.
2
Figure 1: Loss function used in the modeling (median survival)
Results and Discussion
First, the general relationship between the durability of an LLIN product and the number of
nets needed to sustain coverage was explored by calculating the ratio between the number of
LLIN needed in a given year for a 3-year durability LLIN and the corresponding number of LLIN
of a higher durability. This ratio reflects the number of 3-year LLIN needed for each LLIN of
another durability to sustain the target coverage, or – related to the unit cost of the LLIN - by
which factor the more durable LLIN could exceed the price of the 3-year LLIN and still have the
same total cost.
This ratio is independent of population size, the target coverage to be sustained (e.g. 85% as
for PMI or 80% in Global Malaria Action Plan) and the unit cost of the LLIN, but decreases with
an increasing population growth rate. However, these variations are small and at steady-state
(i.e. after 8 years) the ratio for a 7-year LLIN only decreases from 2.00 at 2.34% growth rate to
1.99 at a 2.5% and 1.97 at a 3.5% growth or a 0.5%- to 0.7%-point lower savings in nets.
Based on these findings it is possible to express the potential cost savings of increased
durability in a generic way as:
Savings/LLIN = Cost of 3-year LLIN x ratio 3-year to x-year LLIN – actual cost of x-year LLIN
Table 1 below presents the ratios for a population with an annual growth rate of 2.34% as used
for the later calculations for sub-Sahara Africa. As can be seen, the savings are particularly high
in the second and third year after scale-up and decline thereafter to reach a steady-state after
7-8 years. This phenomenon is caused by the fact that in the model all nets needed to reach
universal coverage are distributed at the same time and initial losses are small but with
relatively higher differences for nets with higher durability (see Figure 1). For a real life
situation where scale-up is achieved over a longer time period the figures for the steady-state
are more realistic. Nonetheless, the ratios indicate that an LLIN with a median durability of 4
years could be 28% more expensive than the 3-year LLIN without increasing the overall cost, a
5-year LLIN 53%, 6-year 77% and a 7-year LLIN 100% more expensive. Similarly, if an LLIN lasts
3
only 2 years on average, the unit cost would have to be only 73% of the 3-year LLIN in order
not to be more expensive.
Table 1: Ratio between the number of 3-year LLIN needed to sustain coverage and an LLIN of
different durability at 2.34% population growth rate (or the unit-cost ratio at equal total cost)
Durability
Year (after achieving target coverage in 2011)
2012
2013 2014 2015 2016
2017
2018
2019
2020
2021
2-year
0.54
0.55
0.66
0.76
0.76
0.72
0.71
0.72
0.73
0.73
3-year
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
4-year
1.48
1.56
1.48
1.37
1.27
1.22
1.23
1.26
1.28
1.28
5-year
1.91
2.17
2.04
1.82
1.62
1.49
1.45
1.47
1.51
1.53
6-year
2.29
2.83
2.70
2.38
2.06
1.85
1.75
1.73
1.75
1.77
7-year
2.59
3.44
3.34
2.95
2.53
2.23
2.07
2.01
2.00
2.00
Figure 2 presents the inverse of these ratios, i.e. number of x-year LLIN needed divided by the
equivalent number of 3-year LLIN and is expressed as % of the 3-year LLIN need. This shows
the potential savings and indicates that at steady-state a 4-year LLIN would require only 78% of
the equivalent need for a 3-year LLIN which means a saving of 22%. The demand for LLIN to
sustain universal coverage then decreases to 65% for a 5-year LLIN, 56% for a 6-year LLIN and
50% for a 7-year LLIN while the demand for a 2-year LLIN would be 38% higher than that of the
3-year LLIN. Interestingly, the marginal gains in savings at steady-state decrease with
increasing durability: 22% saved from a 3- to a 4-year LLIN, 13% from 4- to 5-years, 9% from 5to 6-years and 6% from 6- to 7-year LLIN (total of 50%).
Figure 2: Demand for LLIN of varying durability to sustain universal coverage expressed as proportion of the
numbers for a 3-year LLIN at a population growth of 2.34%
These findings were then applied to the estimated sub-Saharan population at risk of malaria.
Assuming that full universal coverage was reached by end of 2011 the number of LLIN of
varying durability needed to sustain this coverage are shown in Table 2 expressed as millions.
As previously described, the initial need would be small because it is assumed that all LLIN for
scale-up are distributed in 2011 which is not entirely realistic as campaigns are spread out over
4
time not only between countries but in most cases also within so that the cohort of LLIN at
starting point is a mix of different ages of the nets. However, the projections show that
between 130 and 157 million LLIN with a 3-year durability would be required per year for the
sub-Sahara population at risk of malaria to sustain universal coverage assuming the entire
population was targeted for malaria prevention with LLIN. For a 5-year LLIN the demand would
only be between 80 and 100 million and for a hypothetical 7-year LLIN 65-80 million LLIN per
year.
Table 2: Number of LLIN needed per year (in millions) to sustain full universal coverage in subSahara Africa as a function of LLIN durability at a population growth rate of 2.34%
Durability
Year (after achieving target coverage in 2011)
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2-year
77.6 173.0 198.1 190.8 190.4 198.4 203.6 207.4 212.3 217.5
3-year
41.6
95.9 130.2 144.2 145.5 143.8 145.1 149.3 154.0 157.8
4-year
28.1
61.5
87.9 105.4 114.6 117.8 118.2 118.4 120.0 122.9
5-year
21.8
44.2
63.7
79.1
89.9
96.5
99.8 101.2 102.0 102.9
6-year
18.1
33.9
48.3
60.6
70.5
77.9
83.0
86.2
88.1
89.3
7-year
16.1
27.8
39.0
49.0
57.6
64.6
70.1
74.1
77.0
79.0
The next question then is, how much money could be saved with a more durable LLIN. The
unit-cost ratios presented in Table 1 are a generally applicable estimation of potential savings
as they are independent of actual unit cost or whether only the procurement cost are
considered or the cost per net delivered. However, to apply this to the sub-Sahara Africa
situation one has to make some assumptions on the expected price of the nets and in this case
the procurement cost (landed cost) was used. The cost for the 3-year reference LLIN was set at
US$ 4.50 and the marginal cost for each year of durability gained was US$ 0.50 so that the 5year LLIN cost was US$ 5.50 and the 7-year US$ 6.50. This additional cost per year gained could
either be in the procurement of a better material or the cost of a BCC program to increase net
care and repair (or a mix of both). In this scenario the savings per LLIN at steady-state were as
follows (see general formula above):
4-year: US$ 0.76
5-year: US$ 1.39
6-year: US$ 1.97
7-year: US$ 2.50
Figure 3 presents the results with respect to the annual savings for sub-Sahara Africa or
additional cost in the case of the 2-year LLIN. At steady-state the savings range from US$ 100
million per year for a 4-year LLIN and US$ 200 million for a 7-year LLIN. As already described
above (page 3 and Figure 2) the increase in savings per additional durability year gained
becomes smaller with increasing durability.
The cumulative savings are shown in Table 3 and indicate that – if the assumptions underlying
the projections hold – the total savings until the end of 2015 that could be realized for subSahara Africa could range from US$ 439 million for a 4-year LLIN to close to US$ 1 billion for a
7-year LLIN and over the next 10 years could reach between US$ 1 and 2.3 billion.
5
Figure 3: Annual cost savings for sustaining universal coverage in sub-Sahara Africa with increasing LLIN durability
Table 3: Cumulative savings by increased LLIN durability for sustaining universal coverage in
sub-Sahara Africa assuming cost of US$ 4.50 for the 3-year LLIN and a US$ 0.50 marginal cost
per durability year gained
Durability
Year (after achieving target coverage in 2011)
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2-year
-123
-384
-590
-704
-811
-957 -1,119 -1,277 -1,433 -1,593
3-year
0
0
0
0
0
0
0
0
0
0
4-year
46
171
317
439
521
579
640
720
813
908
5-year
67
255
491
705
865
982 1,086 1,201 1,333 1,477
6-year
78
306
603
888 1,120 1,299 1,454 1,609 1,774 1,948
7-year
83
333
666
997 1,278 1,505 1,702 1,892 2,085 2,281
Limitations
A number of limitations must be kept in mind when interpreting these results:
• While it is generally accepted that the best estimate of durability for currently used
LLIN is around three years, the actual data on the performance of different products in
different environments is very limited and the “true” performance of some brands
may be closer to 2-years or even less in some environments and already may reach 4
years or more in others.
• At this point there is no data available on the cost per net life year gained and whether
this would be increasing at a constant rate or exponentially and this is true for the
production cost of more durable nets as well as the cost for behavioral change
programs (BCC) that could increase durability by improved care and repair.
• The scenario modeled here assumed that scale up to universal coverage was achieved
at one point in time in all of sub-Sahara Africa which is not entirely realistic and implies
that the increased early gains following the mass campaign would be smaller in reality.
6
•
However, by only considering the results from the steady-state a conservative
estimate has been presented in the results.
Any potential gains or savings can only be realized if the distribution strategies for LLIN
allow that the longer durability of the LLIN is taken into account and LLIN are not
replaced before they reach the end of their useful life which is achievable, but is likely
to have some limitations. This implies that the actual savings can be expected to be
somewhat lower.
References
1. WHO: Guidelines for monitoring the durability of long-lasting insecticidal mosquito
nets under operational conditions, Geneva, 2011
http://whqlibdoc.who.int/publications/2011/9789241501705_eng.pdf
2. Kilian A: How long does a long-lasting insecticidal net last in the field? Public Health
Journal, 2010, 21: 43-7.
http://www.bayercropscience.com/bcsweb/cropprotection.nsf/id/EN_Public_Health_J
ournal/$file/PHJ_21.pdf
3. Kilian A, Boulay M, Koenker H, Lynch M: How many mosquito nets are needed to
achieve universal coverage? Recommendations for the quantification and allocation of
long-lasting insecticidal nets for mass campaigns. Malaria Journal, 2010; 9:330
7