MALARIA ELIMINATION IN ZANZIBAR - Soper Strategies

those regions from where the greatest numbers of imported cases
in migrants are likely to originate, as depicted in Figure 9(d).
Patterns of malaria in origin regions can be highly seasonal, although
great geographic variation exists in the intensity of transmission and
the amount of the year during which it occurs. By accounting for
the main transmission season months (Tanser et al., 2003) of each
origin region, temporal changes in relative ICR can be examined
to estimate when numbers of imported cases in migrants are likely
to be highest. Estimates of the fraction of infected persons in each
origin region at a given time were combined with migration rates
from each region to estimate the proportion of migrants travelling
to Zanzibar with P. falciparum (Pf) infections over time. Moreover,
the proportion of these ferry passengers traveling to Unguja and
Pemba were assumed to match the ferry capacity proportions
shown in Figure 8, summed over a month.
FIGURE 9
(A) ORIGINS OF MIGRANTS TO ZANZIBAR 2
(C) POPULATION DENSITY PER 100M GRID
SQUARE 4
Estimates
These relative ICR estimates are depicted in Figure 10. They
demonstrate a great range in imported infections risk from
migrants through the year and between islands, reflecting
traffic numbers, passenger origins and transmission seasons
in the regions where passengers originate. Further data on
ferry passenger travel histories is required to confirm ferry
passenger composition, but analyses of mobile phone data
(below) indicate that an overwhelming majority of ferry
passengers simply travel between Zanzibar and Dar es Salaam.
2 Scale shows percentage of total migrants (Gossling and Schulz, 2005)
3 Hay et al., 2009
4 Tatem et al., 2008
(B) P. FALCIPARUM PARASITE PREVALENCE
IN 2007 3
(D) RELATIVE IMPORTED CASE RISK (ICR)
FROM MIGRANTS
HIGH
ICR
LOW
1 | Technical Feasibility
FIGURE 10: RELATIVE IMPORTED CASE RISK (ICR) BY MONTH FROM
MAINLAND MIGRANTS
UNGUJA
PEMBA
JANUARY
FEBRUARY
MARCH
APRIL
MAY
JUNE
JULY
AUGUST
SEPTEMBER
OCTOBER
NOVEMBER
DECEMBER
Control efforts on the mainland will have a significant effect on
imported infection numbers. In the analysis described here, we
have not attempted to adjust for such effects. In reality, however,
the current coverage of interventions like bednets, IRS, and
prompt treatment with ACTs will likely greatly reduce the true
number of imported cases. A careful analysis of the coverage
of these interventions in mainland districts where migrants to
Zanzibar originate will be necessary to assess the magnitude of the
potential reduction; such reductions are likely to be significant
and should increase over time as scale-up campaigns continue in
Tanzania (Figure 11).
FIGURE 11: ITN COVERAGE AMONG CHILDREN UNDER THE AGE OF 5
YEARS PROJECTED TO JULY 2007 (NOOR ET AL., 2009)
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