MALARIA ELIMINATION IN ZANZIBAR - Soper Strategies

NUMBER OF INFECTED INDIVIDUALS
achieved or malaria resurges. Once effective coverage of greater
than 60% of the population is reached, further increases in
coverage will reduce the time required to reach elimination.
Using the model, we can estimate the time required to reach zero
malaria and the predicted paths in Zanzibar under three different
levels of effective coverage (Figure 15, see also Smith and Hay, 2009).
The blue line illustrates a scenario in which every single individual
in Zanzibar is completely protected from mosquito bites–a “best
case” that is extremely unlikely. If such universal protection could
hypothetically be achieved, R C would equal 0, which means that
each case of malaria would produce zero additional cases.
In other words, there would be no ongoing transmission in the
islands. This perfect coverage would produce the fastest path to
achieve elimination. In such a scenario, the expected number of
infected people would decline by approximately 84% each year;
optimistically, fewer than 10 endemic cases would remain in
Unguja after three years, and fewer than 10 endemic cases would
remain in Pemba after four years. As such, even the “best case”
scenario indicates that Zanzibar will not succeed in eliminating
malaria immediately.
FIGURE 15: EXPECTED TIME TO ACHIEVE ELIMINATION UNDER
THREE CONTROL SCENARIOS
10,000
9,000
8,000
7,000
6,000
5,000
4,000
3,000
2,000
1,000
0
ARROWS INDICATE APPROXIMATE YEAR ELIMINATION IS
PREDICTED UNDER EACH LEVEL OF CONTROL
2009 2012 2015 2018 2021 2024 2027 2030
YEARS
COMPLETE CONTROL (RC=0, OR 100% EFFECTIVE COVERAGE)
PARTIAL CONTROL (RC=0.5, OR EFFECTIVE COVERAGE AROUND 75%)
SLIGHT CONTROL (RC=0.75, OR EFFECTIVE COVERAGE AROUND 65%)
In reality, effective coverage is not at 100% and is unlikely ever
to reach it (100% effective coverage would mean that every single
individual was protected by interventions like nets or IRS, and
that the protection conferred by those interventions was absolute).
Figure 15 also depicts two scenarios in which partial effective
coverage is attained. The red line depicts the rate at which
Zanzibar would eliminate malaria if R C=0.5, a transmission level
achievable if effective coverage of approximately 75% is achieved.
Under this situation, elimination could be attained by around
2020. Finally, the green line depicts a scenario in which effective
coverage is only around 65%; such coverage leads to R C of about
0.75 and elimination only after two decades. Effective coverage
levels lower than 65% will be insufficient if Zanzibar chooses to
attempt elimination.
These same models can also be used to evaluate the number
of malaria cases that would occur in Zanzibar due to onward
transmission from inevitable imported cases. To illustrate how
these additional infections would affect the above estimates of
elimination feasibility, it is worth considering the following
example. Suppose 30,000 Zanzibar citizens return to Zanzibar
each month from trips to the mainland and that 1% of them
acquire malaria abroad. At this rate, approximately 300 cases
of malaria will be imported into Zanzibar each month, or
approximately 3,600 a year. If R C = 0.5, there will be an
additional 1,800 cases from onward transmission, for a total of
5,400 new malaria cases each year. This figure would account for
a substantial fraction of the current number of people of malaria
currently infected on the islands, and it would suggest that the
number of imported malaria cases is one of the most critical
numbers to estimate before it is possible to estimate confidently
the likelihood of future progress.
DISCUSSION
1 | Technical Feasibility
According to the most recent ZMCP survey, Zanzibar has
achieved high levels of net use by children under five and pregnant
women (74% and 73% in 2007, respectively). In comparison,
roughly 60% of the general population reported sleeping under a
treated net. In addition, IRS campaigns are reaching even higher
proportions of the population. Of houses visited for the survey,
95.4% had been sprayed with insecticide during the previous six
months. Although data on the overlap between the population
using nets and receiving IRS is not presently available, the
combination of these two control activities may result in higher
overall effective coverage.
If such high IRS coverage levels can be maintained, and if they
can remain effective, these exceptional levels of coverage could
put Zanzibar on a course to follow a trajectory more similar to
the red or blue curves in Figure 10 than the slow green one, and
elimination could be achieved within the next decade. However,
without the coverage by IRS, effective coverage would revert to
the approximately 60% general coverage by ITNs. As discussed
above, 60% coverage probably equates to an R C value of close to
one; as such, elimination will not be achieved at these levels.
Because the model does not discriminate at present between
nets and spraying, the outputs described here indicate that the
higher IRS coverage renders the lower net coverage irrelevant. It
would be a mistake to conclude from this apparent redundancy
that bed nets could be scaled back as long as IRS is maintained.
For one thing, development of resistance or behavioral change
in mosquitoes could greatly reduce the effective coverage by
IRS. Without the additional protection of nets, loss of effective
coverage due to resistance to IRS would result in a high R C and
subsequent risk of a rapid resurgence in transmission.
Additionally, there is considerable heterogeneity in both
malaria risk and intervention coverage on Zanzibar that must
be considered when examining the potential for elimination
25