A comparison of Markov and mechanistic models for soil-transmitted helminth (STH) prevalence projections in the context of survey design

Globally, there are over one billion people infected with soil-transmitted helminths (STHs), mostly living in marginalised settings with inadequate sanitation in sub-Saharan Africa and Southeast Asia. The WHO recommends an integrated approach to STH morbidity control through improved access to sanitation and hygiene education, and the delivery of preventive chemotherapy (PC) to school age children delivered through schools. Progress of STH control programmes is currently estimated using a baseline (pre-PC) school-based prevalence survey and then monitored using periodical school-based prevalence surveys, known as Impact Assessment Surveys (IAS). We investigated whether integrating geostatistical methods with a Markov model or a mechanistic transmission model for projecting prevalence forward in time from baseline can improve IAS design strategies. To do this, we applied these two methods to prevalence data collected in Kenya, before evaluating and comparing their performance in accurately informing optimal survey design for a range of IAS sampling designs. We found that although both approaches performed well, the mechanistic method more accurately projected prevalence over time and provided more accurate information for guiding survey design. Both methods performed less well in areas with persistent STH hotspots where prevalence did not reduce despite multiple rounds of PC. Our findings show that these methods can be useful tools for more efficient and accurate targeting of PC. The general framework built in this paper can also be used for projecting prevalence and informing survey design for other NTDs.