MALARIA ELIMINATION IN ZANZIBAR - Soper Strategies
MALARIA ELIMINATION IN ZANZIBAR - Soper Strategies
MALARIA ELIMINATION IN ZANZIBAR - Soper Strategies
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PROBABILITY OF ACQUIR<strong>IN</strong>G <strong>IN</strong>FECTION<br />
FIGURE 13: ALL TRIPS MADE BY <strong>ZANZIBAR</strong> RESIDENTS PLOTTED BY<br />
PROBABILITY OF <strong>IN</strong>FECTION ACQUISITION, BASED ON REGION<br />
POPULATION WEIGHTED MEAN dEIR (RED L<strong>IN</strong>E) AND POPULATION<br />
WEIGHTED PR<strong>IN</strong>CIPAL CITY MEAN dEIR (BLUE L<strong>IN</strong>E)<br />
250000<br />
250000<br />
NUMBER NUMBER OR OR TRIPS<br />
100000 150000 200000<br />
100000 150000 200000<br />
50000<br />
50000<br />
00<br />
0.0 0.2 0.4 0.6 0.8 1.0<br />
0.0 0.2 0.4 0.6 0.8 1.0<br />
PROBABILITY OF AQUIR<strong>IN</strong>G <strong>IN</strong>FECTION<br />
PROBABILITY OF ACQUIR<strong>IN</strong>G <strong>IN</strong>FECTION<br />
FIGURE 14: BOXPLOTS OF TRIP PROBABILITIES OF <strong>IN</strong>FECTION<br />
ACQUISITION BY ZANTEL REGION UNDER SCENARIOS OF (A) REGION<br />
POPULATION WEIGHTED MEAN dEIR; (B) POPULATION WEIGHTED<br />
PR<strong>IN</strong>CIPAL CITY MEAN dEIR<br />
0.7 (A) 0.7 (B)<br />
PROBABILITY OF ACQUIR<strong>IN</strong>G <strong>IN</strong>FECTION<br />
0.6 0.6<br />
0.5 0.5<br />
0.4 0.4<br />
0.3 0.3<br />
0.2 0.2<br />
0.1 0.1<br />
0.0 0.0<br />
ARUSHA DAR DODOMA MBEYA MWANZA ARUSHA DAR DODOMA MBEYA MWANZA<br />
Figure 14 shows the regional composition of the distributions<br />
in Figure 13, illustrating that under both scenarios, the trips<br />
made by residents to Dodoma, Mbeya and Mwanza provided<br />
greater risks of infection acquisition, due to a higher proportion<br />
of longer stays in these regions typically, combined with overall<br />
high levels of transmission.<br />
By summing the probabilities of infection for all the trips made,<br />
estimates of imported infection numbers from returning residents<br />
can be made. Under the two dEIR transmission level scenarios<br />
presented here, this resulted in estimates of between 105 and 1,243<br />
imported cases per month on average (though according to Figure<br />
8, the months used are higher travel months on average), equating<br />
to a range of 1-12 cases per 1,000 residents per year.<br />
OPPORTUNITY FOR ONWARD TRANSMISSION<br />
The above calculations provide estimates of the number of infected<br />
individuals traveling to Zanzibar. However, if those individuals<br />
spend a single afternoon in a Stonetown hotel, they are very<br />
unlikely to infect a mosquito during their time in Zanzibar. A<br />
person who brings malaria into the country and stays for a week<br />
1 | Technical Feasibility<br />
is equivalent to seven people who stay overnight. The destination<br />
of travelers, their length of stay, and their lodging and activities<br />
while there will all greatly influence the overall importation risk.<br />
As such, the importation risk is a priori very different for different<br />
groups of people. Zanzibar residents, immigrants, and migrant<br />
workers are likely to stay on the islands the longest after acquiring<br />
an infection. They are also most likely to travel to areas of the<br />
islands with high transmission risk and to engage in activities<br />
that are conducive to transmission (sleeping in open areas rather<br />
than air conditioned rooms). If an infected individual is treated<br />
promptly, their infection will be cleared and they will be less likely<br />
to spread the infection. However, treatment seeking behavior<br />
with all of these groups will be an added challenge; in many cases,<br />
these people will have acquired malaria immunity, so they have<br />
mild or no symptoms and will be less likely to seek treatment.<br />
Treatment-seeking practices are known to vary by socioeconomic<br />
status (McCombie, 1996), so it is likely that migrant individuals<br />
with lower incomes will be less likely to be treated with effective<br />
ACTs than other migrant groups.<br />
DISCUSSION<br />
The calculations performed for migrant ICR in this section have<br />
relied upon data on travelers entering Zanzibar by ferry. While<br />
it is reasonable to assume that air passengers contribute little<br />
to importation risk, it is possible that informal traffic, including<br />
traders moving between the mainland and islands on small boats,<br />
could present a significant risk. Little information is available on<br />
this informal traffic but assuming that these informal travelers<br />
have a similar mobile phone use profile compared to the general<br />
population, the mobile phone analyses likely incorporate informal<br />
travel.<br />
Assuming that returning Zanzibar residents represent half of all<br />
travelers to the island, and that visitors from the mainland have<br />
similar travel patterns (evidence from the mobile phone data<br />
suggests that this may be true), estimates from the mobile phone<br />
data analyses provide a likely range of 2-24 infections per 1000<br />
inhabitants per year. However, actual importation risk will likely<br />
be towards the lower end of this range due to many individuals not<br />
spending sustained time on the islands, not traveling to areas of<br />
high transmission risk (urban areas), or not coming into contact<br />
with vectors. Moreover, residents traveling to the mainland may<br />
sleep under a net. For the modeling components below, we thus<br />
estimate importation to range from 2-8 infections per 1000<br />
inhabitants per year. However, improved information on all<br />
these factors will be essential to designing an effective program<br />
to prevent reintroduction of malaria following elimination. In<br />
particular, data must be collected on:<br />
�� How many individuals travel to Zanzibar by ferry or informal<br />
boats (including visitors and returning residents)?<br />
�� Where do these travelers originate?<br />
�� Where is their destination in Zanzibar?<br />
�� How long are they likely to stay there?<br />
�� What is their socioeconomic status and lodging while on the<br />
islands?<br />
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