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