The challenge of one billion adjuvanted vaccine doses: evaluating scalability, sustainability, and supply capacity of Quillaja saponin QS-21 for large-scale vaccine demand

biomass sourced in Chile. This review summarizes the historical development, chemical features, and immunological relevance of QS-21, and critically evaluates existing and emerging production technologies in terms of a projected production benchmark of 50 Kg per year (corresponding to at least one billion 50 µg doses). We conclude that appropriate management of wild Quillaja forests can sustain kilogram-scale QS-21 purification, but to meet increasing demand, clonal plantation forestry will be the most practical and scalable strategy in the near future. Alternative approaches, including plant cell culture, engineered microorganisms, and chemical synthesis, remain scientifically promising but currently face technical and economic limitations for large-scale pharmaceutical production.