Pre-clinical studies of Schistosoma mansoni vaccines: a scoping review
BackgroundSchistosomiasis is caused by infection with worms of the genus Schistosoma including S. mansoni. Over 200 million people are infected, sterile immunity does not naturally develop, and no vaccine is available. A vaccine could be a critical tool to achieve control and elimination. Numerous candidates have been tested in pre-clinical models, but there is not yet an approved vaccine. Methodology/Principal FindingsWe conducted a scoping review using a keyword search on Web of Science and a MeSH term search on PubMed. Articles were screened and included if they tested a defined vaccine candidate in a pre-clinical protection assay against S. mansoni between 1994-2024. Vaccine formulation, study design, and efficacy parameters from all articles were extracted. This data was summarised graphically, with the influence of different parameters appraised. A total of 141 candidate antigens were tested in 108 articles over the last 30 years, with most antigens tested only once and three (Sm-CatB, Sm-p80, and Sm-14) tested over 20 times. The median protective efficacy against worms was 35%. 10 antigens achieved over 60% efficacy, and only two (Sm-p80 and Sm-CatB) over 90%. Large variations in efficacy were observed with all repeatedly tested antigens, likely attributable to differing formulations and study designs. The effect of these varying parameters on the resultant efficacy was evaluated. ConclusionsA few vaccine candidates have achieved promising efficacy in pre-clinical studies. Most vaccines tested however have efficacy that falls short of that required for an impactful schistosomiasis vaccine. The diversity in study designs makes comparing vaccine targets a challenge. Use of consistent and optimized vaccine formulation (including adjuvant and platform) and study design parameters is critical to expedite the development of a schistosome vaccine. Author SummarySchistosomiasis, a major neglected tropical disease, is caused by infection with parasitic worms of the Schistosoma species, including Schistosoma mansoni. Individuals can be repeatedly re-infected, and there is no available vaccine. An initial stage of vaccine development is testing in a pre-clinical animal model. Here we have summarised tests of Schistosoma mansoni vaccines in the last three decades. 100+ vaccine candidates have been tested, with only 10 of these achieving efficacy of over 60%, and only 2 at over 90%. When the same vaccine candidate (antigen) is tested in a different formulation, or using different study design the efficacy varies greatly. We have summarised the formulations and study designs used, and highlighted how certain parameters affect efficacy. Finally, we have assembled a series of recommendations to researchers on how to perform vaccine tests in the future.