risk, versus a 0.86 reduction in risk when concentrating on the areas where the top 25% of sewer bursts are concentrated. The efficiency and cost effectiveness of reducing cholera risk is therefore much greater when concentrating areas that fall within the top 25th percentile of sewer bursts. It is also more efficient to provide piped water to within 500 meters of the existing network (7.55), rather than to extend access to the whole city (8.2) when considering their relative impact on reducing the number of cholera cases.

Finally, we also see how extending access to the sewer network in its state at the time of the data collection in fact seems to increase cholera risk. This finding may seem perverse given that according to the Joint Monitoring Program of the WHO, which set the standards for sanitation globally, sewered sanitation is at the top of the improved sanitation ladder. However, it serves to highlight the importance of proper maintenance of existing sanitation networks rather than necessarily emphasizing new construction. This is particularly important in a context where the institutional capacity and financial resources to maintain large-scale sanitation systems may be lacking.

An important limitation of the study is that the analysis presented relies on observational data and thus we draw correlations and associations, rather than drawing causal inferences. This clearly has important implications when interpreting the scenario analysis. Additionally, there are likely to be some errors in case data such as the locations and times of case identification. Similarly, a number of layers used in the analysis, such as proportion of households with access to piped water, were generated using DHS data, which have been spatially obfuscated. As a result, this may have introduced error into the layer, which could obscure, or falsely lead to, associations with cases. In addition, sewer and water burst density was based on spatiotemporally referenced complaints data, which is obviously a proxy measure with a number of reported and recorded issues. Data from direct inspections of sewer and water networks would have been a more reliable measure but such data was not available to the researchers.

The focus of this analysis has been to study what factors might be associated with a cholera outbreak and could explain the spatial heterogeneity in the outbreak. An area of future research would be to explore what may lead to some of these factors, especially sewer bursts, in order to be able to predict areas at risk for future outbreaks. Above all, this analysis shows the importance of emphasizing maintenance above and beyond new construction, especially when thinking of extending an already poorly maintained system.

# Acknowledgments

The authors acknowledge with thanks the financial support received from the Global Water Security and Sanitation Partnership (GWSP). The authors also acknowledge the cooperation of officials from City of Harare (CoH) Water and GIS Departments, in particular Toine Ramaker from VEI, Dr. Manangazira from the Ministry of Health, Zimbabwe; in country counterparts at the World Health Organization (WHO); and the Country Management Unit (CMU) of the World Bank in Zimbabwe, in particular then Country Manager Mukami Kariuki, Senior Operations Officer Tonderai Fadzai Naome Mukonoweshuro and Health Specialist Consultant Chenjerai Sisimayi for facilitating contact with counterparts and local dissemination of findings.

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