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Objective To determine if the prevalence of schistosomiasis in children aged 9­12 years is associated with the prevalence in 5­8-year-olds and adults after preventive chemotherapy in schools or the community. Methods We combined data from four community-randomized, preventive chemotherapy trials in treatment-naïve populations in Côte d'Ivoire, Kenya and the United Republic of Tanzania during 2010­2016 according to the number of praziquantel treatments and the delivery method. Schistosoma mansoni infection was sought on two slides prepared from each participant's first stool using the Kato­Katz technique. We assessed associations between S. mansoni prevalence in 9­12-year-olds and 5­8-year-olds and adults in the community before and after treatment using Bayesian regression models. Findings Stool samples from 47 985 5­8-year-olds, 81 077 9­12-year-olds and 20 492 adults were analysed. We found associations between the prevalence in 9­12-year-olds and that in 5­8-year-olds and adults after preventive treatment, even when only school-age children were treated. When the prevalence in 9­12-year-olds was under 10%, the prevalence in 5­8-year-olds was consistently under 10%. When the prevalence in 9­12-year-olds was under 50%, the prevalence in adults after two or four rounds of preventive chemotherapy was 10%­15% lower than before chemotherapy. Post-chemotherapy age-group associations were consistent with pre-chemotherapy associations in this analysis and previous studies. Conclusion The prevalence of S. mansoni infection in 9­12-year-olds was associated with the prevalence in other age groups and could be used to guide community treatment decisions.

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