Previous geostatistical work

Trachoma is targeted for elimination by 2030. To achieve this, health ministries must demonstrate that active trachoma prevalence is below 5%, trachomatous trichiasis (TT) is below 0.2%, and there is a system in place able to identify and manage incident TT cases. While progress has been made, some districts continue to show persistent or recrudescent trachoma. Model-based geostatistics (MBG) is a valuable tool for disease risk assessment in resource-constrained settings. It predicts prevalence at specific locations by considering spatial factors, enabling more efficient monitoring and evaluation. Working with Lancaster University, we explored the possibility of its use to design more efficient surveys and assess the likelihood that true prevalence is below the target threshold.

It is suggested that MBG can offer three potential advantages for trachoma programmes:

Efficient Survey Design: MBG leverages spatial correlations to design more cost-effective Trachoma Impact Surveys (TIS) or Trachoma Surveillance Surveys (TSS). For example, MBG has been used to design super-EUs, where multiple EUs are combined into one, allowing fewer clusters to be surveyed while maintaining the same level, or even improved precision.

Improved Interpretation: MBG analysis provides statistical certainty about whether a district has achieved the elimination threshold. This helps programme implementers differentiate meaningful prevalence changes from sampling error.

Targeted Interventions: MBG identifies geographic areas with a high probability of exceeding prevalence targets, allowing more focused resource allocation. For example, it can aid in deciding whether to split districts, or whether to discontinue (MDA) in areas where the prevalence threshold has been met, while directing efforts to where it has not.

During the grant we held a webinar to give an overview of MBG and the potential use cases, this took place in August 2023 and can be watched here. Speakers gave presentations on the background, technical implementation and rationale for using this approach, and two case-studies were presented by health ministry partners. The webinar slides can be found here. 

From 4–5 March 2024, a technical consultation was convened by the WHO Collaborating Center on Geostatistical Methods for NTD Research at Lancaster University, UK, to:

Background information was provided on the rationale for using MBG to assess trachoma elimination, including case studies demonstrating improved precision of TT prevalence estimates and the wide applicability of MBG in a range of epidemiological settings. Factors that need to be considered before MBG can be implemented on a wider scale were also presented to attendees to encourage discussion and identify the next steps required to progress with using MBG for trachoma elimination purposes.

The discussions amongst attendees highlighted important research questions that need to be addressed before MBG can be offered as a standardised survey design and analysis tool for trachoma surveys. Therefore, additional support will be required to take this forward. Further technical workshops will also be critical to make decisions regarding risk thresholds and appropriate use cases of MBG. At present, Tropical Data cannot routinely support MBG. However, anyone interested in participating in trachoma-MBG research activities can contact Lancaster University directly (e.giorgi@lancaster.ac.uk). A full report of the technical consultation meeting is due to be published in 2025. 

To aid understanding of MBG, a glossary of relevant terms can be found here.

Finally, a list of frequently asked questions developed during the grant can be found here.