RESEARCH Table 2. Baseline characteristics, estimated coefficients, and standard errors for the model Epidemic experience (n districts)a Variable Ever (%) Never Absolute humidity profileb Class 1 15 (3) 527 Class 2 9 (6) 153 Class 3 19 (8) 228 Class 4 43 (47) 48 Class 5 143 (61) 93 Class 6 0 (0) 2 Class 7 118 (47) 132 Class 8 40 (28) 102 Class 9 106 (48) 117 Class 10 46 (22) 161 Class 11 16 (29) 39 Class 12 1 (11) 8 Class 13 90 (48) 97 Class 14 44 (52) 40 Class 15 178 (74) 64 Class 16 7 (37) 12 Class 17 25 (54) 21 Class 18 181 (80) 46 Class 19 105 (56) 84 Class 20 46 (39) 73 Land-cover type Savanna 646 (39) 1006 Dryland cropland/pasture 44 (39) 68 Irrigated cropland/pasture 0 (0) 9 Cropland/grassland mosaic 198 (53) 174 Cropland/woodland mosaic 4 (4) 105 Grassland 123 (67) 61 Shrubland 66 (34) 127 Urban 3 (43) 4 Broadleaf deciduous forest 44 (39) 68 Evergreen broadleaf forest 37 (13) 248 Water bodies 20 (38) 33 Forest wetland 0 (0) 24 Barren/sparsely vegetated 46 (28) 120 Variable removede Log likelihood Change in –2 log likelihood Absolute humidity profile –2045.937 877.963 Land-cover type –1642.564 71.217 aExcludes five districts for which environmental data were unavailable (in Sinai, Egypt, and Pemba, Tanzania). b See Figure 1 for description of profile classes. cp value <0.01. dp value <0.05. eModel if term removed. β Multivariable analysis SE Reference 0.59 1.00c 3.48c 3.97c –2.44 3.29c 2.40c 3.31c 2.12c 3.34c 2.17d 3.30c 4.00c 4.46c 3.61c 3.99c 4.82c 3.60c 3.45c –0.38 –7.02 –0.07 –1.97c 0.36 –0.52c 0.32 0.17 0.07 –0.07 –2.82 –1.07c df 19 12 0.44 0.36 0.35 0.31 15.73 0.30 0.33 0.31 0.32 0.43 1.13 0.32 0.36 0.32 0.59 0.43 0.32 0.31 0.35 Reference 0.23 7.14 0.14 0.56 0.19 0.19 0.85 0.23 0.24 0.33 4.55 0.24 Significance of change <0.001 <0.001 factor associated with the distribution of epidemics was humidity. Areas without a marked distinction between wet and dry seasons were less likely to have had epidemics than those with contrasting seasons. The areas without dis- tinction between wet and dry seasons include deserts and the humid parts of coastal and central Africa, much of which are forested, and the areas with contrasting seasons comprise the semiarid savannah and grasslands of the Sahel and east and southern Africa. Surface maps of Africa demonstrated a close correspondence between humidity and land-cover types in these regions. The model also showed that, having accounted for the effects of humidity, sparsely vegetated and barren regions, areas of woodland mosaic, and shrub land were less likely than other regions to have ever had an epidemic. The Sahel, which has a pro- longed dry season with low humidity was identified as the area with the greatest risk (p>0.6). Peripheral regions along its southern borders, where the dry season is shorter and less extreme, carry a moderate risk (p>0.4). The peripheral region extends from southern Sudan and Ethiopia to the Great Lakes and Rift Valley regions and parts of southern Africa peripheral to desert areas. 1290 Emerging Infectious Diseases • Vol. 9, No. 10, October 2003