18 FIGURE 5: <strong>MALARIA</strong> TEST POSITIVITY RATES BY DISTRICT FOR (A) 2008 AND (B) 2009 Unguja Urban 1 - 2.1% 1 - 2.1% North B 4.5 - 5.5% West 1 - 2.1% North A 1 - 2.1% Central 5.6 - 6.6% South 2.2 - 3.2% Pemba Micheweni 3.3 - 4.4% Wete 2.2 - 3.2% Chakechake Mkoani 1 - 2.1% 1 - 2.1% Malaria Epidemic Early Detection System, Positivity Rates (%) in 52 Health Facilities Unguja Urban 0.6 - 1.7% 0.6 - 1.7% North B 4.9 - 5.9% West 0.6 - 1.7% North A 0.6 - 1.7% 2.2 - 3.2% 3.3 - 4.4% 4.5 - 5.5% 5.5 - 6.6% Central 4.9 - 5.9% South 1.8 - 2.7% Pemba Micheweni 4.9 - 5.9% Wete Chakechake Mkoani 0.6 - 1.7% 0.6 - 1.7% 1.8 - 2.7% Malaria Epidemic Early Detection System, Positivity Rates (%) in 52 Health Facilities 1.8 - 2.7% 2.8 - 3.8% 3.9 - 4.8% 4.9 - 5.9% (A) 2008 (B) 2009
DEF<strong>IN</strong><strong>IN</strong>G IMPORTATION RISK As illustrated in Figure 5, the control measures that are required to keep overall malariogenic potential in check and thus ensure sustainable elimination will differ depending upon the level of importation risk that exists. For this reason, understanding the amount of malaria being transported to the islands is as essential as understanding the risk of onward transmission. Importation risk involves the probability of infectious individuals or mosquitoes carrying malaria parasites into Zanzibar. Because Zanzibar is comprised of islands, the risk of importing malaria is far higher in human hosts than mosquitoes, and we ignore the latter risk here. Calculating importation risk quantitatively involves considering each of the following factors: �� The entry routes through which people travel to Zanzibar, such as ferry, airplane, or informal boats; �� The number of people who travel into Zanzibar through each of these entries by traveler type, such as tourists, migrant laborers, or Zanzibar citizens; �� The probability of travelers being infected with malaria parasites, which will vary with the traveler type and where they stayed prior to arrival in Zanzibar; returning Zanzibar residents will have different probabilities of bringing parasites back with them depending on where on the mainland they stayed; and �� The probability of infected individuals within those groups infecting a mosquito in Zanzibar, which depends upon how long and where they stay. In this section, we consider approaches to quantifying each of these factors, though limited data availability increases the uncertainty surrounding the estimates derived from information obtained on air and ferry passenger numbers, and evaluations on tourism to Zanzibar. A novel approach using mobile phone use data obtained from the Zantel mobile phone company allowed for more precise estimates, especially for Zanzibari residents traveling to the mainland. Careful assessment of each component must occur during the next several years as Zanzibar plans its strategy for eliminating malaria and ensuring that it does not return. IMPORTATION RISK ANALYSIS BASED ON ROUT<strong>IN</strong>E TRAVEL/ TOURISM DATA Routes of Importation Because Zanzibar is comprised of islands, the majority of incoming people and consequently, parasites, arrive through specific gateways, including ferry ports and airports. A country sharing many land borders with neighbors would have far more points of entry, greatly complicating these calculations. Records exist of some passenger traffic via ferries and airplanes; however, no statistics exist at present on the specific numbers of passengers involved in informal boat traffic between the islands and the mainland (although landing points are known), so future surveys and surveillance on informal movements will be necessary to refine migration estimates. 40,000 35,000 30,000 25,000 20,000 15,000 10,000 5,000 1 | Technical Feasibility Air Transport As tourism to Zanzibar continues to increase, so do the number and capacity of incoming flights (Tanzania Ministry of Natural Resources and Tourism, 2007). Figure 6 shows that December- January and July-August represent the peak incoming passenger seasons, and that numbers increased significantly over the course of 2006 and 2007. This increase does not necessarily indicate an accompanying growth in infection importation risk because many, if not most, of the flights may be carrying tourists (often taking prophylactic drugs) traveling from non-endemic countries who have had to change flights in Malaria Endemic Countries (MEC) to get to Zanzibar (see tourism section below). As such, while data on passenger origins and purpose of visit are required for precise estimates, it can be safely assumed that air travel represents a minority of the risk of parasite importation. FIGURE 6: NUMBERS DISEMBARK<strong>IN</strong>G <strong>IN</strong> <strong>ZANZIBAR</strong> FROM DOMESTIC FLIGHTS (TANZANIA MA<strong>IN</strong>LAND <strong>IN</strong>CLUDED) <strong>IN</strong> 2006 AND 2007 J F M A M J J A S O N D J F M A M J J A S O N D MONTHS Ferry Transport Through collation and analysis of available ferry timetable data, Figure 7 shows the major ferry routes and daily capacities. These figures give a useful indication of maximum expected numbers of visitors and movements between islands, but do not indicate where passengers have typically traveled from. Nevertheless, the data shown in Figure 7 provide useful figures for estimating the volume and location of entry of passengers entering Zanzibar through the ferry system. Figure 8 provides additional information on actual numbers of ferry passengers and how these have varied through 2006 and 2007. The data shows some seasonal fluctuations and also some significant inter-annual variations. However, this data does not distinguish between numbers arriving or leaving the islands, which islands are involved and where passengers have come from–these are all features that should be included in future surveys. 19
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