We reanalyzed data from a phase III trial for the killed oral cholera vaccine to test two hypotheses: there will be a greater impact of the vaccine in areas where there is a low force of infection, and the spatial pattern of disease transmission will change after a mass vaccination campaign. Spatial regression was used to test these hypotheses accounting for spatial heterogeneity in disease and vaccine coverage. The results of the analyses confirm both hypotheses. The paper also shows how spatial analysis can be used to understand the impact of vaccination when there are spatially heterogeneous disease distributions.