Triterpene compound QS-21 is an increasingly important adjuvant or immune booster used alongside vaccines, including for Covid-19, shingles and malaria. Traditionally extracted from the bark of Quillaja saponaria, a tree native to Chile, prices of QS-21 have recently skyrocketed due to laws protecting against deforestation and other factors, says Gaston Salinas, CEO of US-based Botanical Solution (BSI) - to ‘around $200,000 to $400,000/g’.