RESEARCH Figure 2. Age-adjusted optical density (OD) values for antibodies against Plasmodium falciparum in the study population, Ceel- Bardaale, Somalia, 2008. Colored dots indicate mean age-adjusted optical densities per household for combined seroreactivity to P. falciparum merozoite surface protein 119 and apical membrane antigen 1. The large circle indicates a statistically signiﬁcant cluster of higher P. falciparum seroreactivity that was detected by a spatial scan on the age-adjusted seroreactivity of individual study participants to both P. falciparum antigens (p = 0.002). As a result of age adjustment, some persons had lower than expected seroreactivities. This adjustment resulted in negative OD values. P. falciparum and 1 with higher seroreactivity against P. vivax. In Ceel-Bardaale, where households were scattered along a delta of seasonal rivers, antibody prevalences to P. falciparum and P. vivax were negatively associated with distance to the nearest river. In several areas of higher en- demicity, distance to the nearest body of water has been related to malaria incidence (5,20,31,32) and immune re- sponses (20,32). No other factors were signiﬁcantly related to malaria-speciﬁc immune responses. Our data indicate that serologic markers can be used to determine variation in transmission intensity at levels of malaria transmission that are too low for sensitive as- sessments by microscopy, RDT, or entomologic tools. The sensitivity of serologic analysis to detect small-scale differences in transmission intensity may prove extreme- ly useful in evaluating malaria control programs in areas where conventional malariometric markers fail. It may also provide vital information on which areas are most likely to be receptive to transmission if malaria epidemics were to occur. Acknowledgments We thank Abdikarim Yusuf for training and supervising of laboratory technicians; Fahim Yusuf and Ahmed Noor for facili- tating training of surveyors and supervising ﬁeld work; Tanya Shewchuk, the United Nations Children’s Fund (UNICEF)–So- malia, the World Health Organization, UNICEF country ofﬁces, the Kenya Medical Research Institute (KEMRI), Wafaa Said, Ahmed Mohamed Jama, and Lydiah Mwangi for administrative support; Moses Mosobo and Ken Awuondo for technical support for microscopy; Bart Faber for providing P. vivax AMA-1 anti- gen; Shona Wilson, Patrick Corran, Lynn Spencer, Eleanor Riley, and Immo Kleinschmidt for support and advice; the director of KEMRI for support; and the clan elders, homestead heads, and community members for participating in the survey. This study was supported by The Global Fund to Fight AIDS, Tuberculosis and Malaria to UNICEF–Somalia (YH/101/04/08). T.B. is supported by a Rubicon fellowship of the Netherlands Or- ganisation for Scientiﬁc Research (825.05.025). J.C. (078925), C.D. (078925), A.M.N. (081829), and R.W.S. (079081) are sup- ported by the Wellcome Trust UK. Dr Bousema is a lecturer in the Department of Infectious and Tropical Diseases at the London School of Hygiene and Tropical Medicine. His research interests include malaria immune epide- miology and transmission-reducing strategies. References 1. Guerra CA, Gikandi PW, Tatem AJ, Noor AM, Smith DL, Hay SI, et al. The limits and intensity of Plasmodium falciparum transmission: implications for malaria control and elimination worldwide. PLoS Med. 2008;5:e38. DOI: 10.1371/journal.pmed.0050038 2. Snow RW, Omumbo JA, Lowe B, Molyneux CS, Obiero JO, Palmer A, et al. Relation between severe malaria morbidity in children and level of Plasmodium falciparum transmission in Africa. Lancet. 1997;349:1650–4. DOI: 10.1016/S0140-6736(97)02038-2 3. Hay SI, Smith DL, Snow RW. Measuring malaria endemicity from intense to interrupted transmission. Lancet Infect Dis. 2008;8:369– 78. DOI: 10.1016/S1473-3099(08)70069-0 4. Nájera JA. Malaria control, achievements, problems and strategies. Report no. WHO/MAL/99.1087. Geneva: World Health Organiza- tion; 1999. 5. Oesterholt MJ, Bousema JT, Mwerinde OK, Harris C, Lushino P, Masokoto A, et al. Spatial and temporal variation in malaria trans- mission in a low endemicity area in northern Tanzania. Malar J. 2006;5:98. DOI: 10.1186/1475-2875-5-98 6. Smith T, Charlwood JD, Takken W, Tanner M, Spiegelhalter DJ. Mapping the densities of malaria vectors within a single village. Acta Trop. 1995;59:1–18. DOI: 10.1016/0001-706X(94)00082-C 398 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 16, No. 3, March 2010