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. 2021 Feb 24;13(2):170-177.
doi: 10.1093/inthealth/ihaa042.

Spatial clustering of cholera cases in the Kathmandu Valley: implications for a ring vaccination strategy

Mellisa Roskosky  1 , Mohammad Ali  1 , Shyam Raj Upreti  2 , David Sack  1
Affiliations

Spatial clustering of cholera cases in the Kathmandu Valley: implications for a ring vaccination strategy

Mellisa Roskosky et al. Int Health. .

Abstract

Background: In mid-2016, a cholera outbreak occurred in Kathmandu Valley, Nepal. This retrospective study aims to determine if a reactive, ring vaccination strategy would have been useful in preventing cholera transmission during that outbreak.

Methods: Data on cholera cases were collected as part of hospital-based surveillance in the Kathmandu Valley in 2016. Global Positioning System (GPS) coordinates were obtained during household visits. Geographic clusters of cases were visually determined and tested statistically for clustering. Cluster size was determined based on the distribution of cases around the index case.

Results: GPS coordinates for 69 cases were analysed. Six geographic clusters were identified, all of which showed significant clustering of cases. Approximately 85% of cases within a cluster occurred more than 7 d after the index case. The median ring size was 1 km, with a population of 14 000 people.

Conclusions: Cholera cases were clustered in space and the majority of cases occurred over 1 week after the initial cases in the cluster, allowing for an opportunity to prevent transmission through the use of the vaccine soon after the initial case was identified. A ring vaccination strategy may be especially useful for large urban areas with recurrent seasonal outbreaks but where the specific locations for such outbreaks are not predictable.

Keywords: Nepal; cholera; oral cholera vaccine; ring vaccination.

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Figures

Figure 1.
Figure 1.
Study area, sentinel site hospitals and distribution of cases during the 2016 cholera outbreak.
Figure 2.
Figure 2.
Epidemic curve of the cholera outbreak in Kathmandu Valley, 2016. Cases are shown per week. Curve includes all 193 confirmed cases during the outbreak.
Figure 3.
Figure 3.
Data flow chart. A total of 69 cases were included in the spatial analysis.
Figure 4.
Figure 4.
Diffusion of cholera in space and time in the different geographic clusters in Kathmandu Valley, 2016. Triangles indicate a cluster index, along with the cluster number. Each point represents a case, with lines indicating potential transmission over time.
Figure 5.
Figure 5.
K and L functions for global spatial clustering (based on 69 cases). Black lines indicate the degree of clustering that was observed in the data. Red lines indicate what would be expected if cases were completely spatially random. The grey envelope represents the 95% confidence interval.
Figure 6.
Figure 6.
Spatial and temporal dynamics of cholera cases by cluster. Distance and time are relative to the cluster index case.
See this image and copyright information in PMC

References

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