Our findings are likely to be important in vaccine development and introduction. Shigellosis still causes a considerable burden in Kaengkhoi. The observed fluctuations in incidence during the three-year surveillance period, including a cluster of cases in summer 2001, indicate the potential for outbreaks as have been previously described in Thailand (14, 16). Any vaccine designed to protect against shigellosis in this region of Thailand should protect against S. flexneri, which has caused more hospitalizations in the past, as well as S. sonnei, which caused the majority of shigellosis cases in this study.The dominant S. flexneri serogroups are 1b, 2a and 3b. It is uncertain whether cross-protection exists between Shigella serogroups, thus there may be a need for a polyvalent vaccine to protect against S. flexneri 1b, 2a, and 3b as well as S. sonnei. O