The inner bark of the Chilean soapbark tree, Quillaja saponaria, is the source material for some of these saponins. Pulverized and soaked in water at the Desert King factory in Chile, the bark is transformed into a brown, bitter, bubbly fluid. This precious goo does many things well, and it happens to be the raw material for one of the world’s most coveted vaccine adjuvants: QS-21. Adjuvants are compounds that boost the body’s immune reaction to a vaccine. Owing to their potential risks to human health, however, only a handful of adjuvants have been approved by the U.S. Food and Drug Administration, and QS-21 is one of the newest…

or all the talk about the cutting-edge vaccines that may just get us out of the COVID-19 mess, little has been written about adjuvants. Perhaps that shouldn’t be surprising: The late Yale professor Charles Janeway famously called adjuvants the “immunologist’s dirty little secret.”

These unheralded helpers can turn a half-baked vaccine into an effective one, or stretch a scarce vaccine supply during a pandemic. Not every vaccine requires an adjuvant, but many do: Of the more than 200 vaccines listed in the Milken Institute’s COVID-19 vaccine tracker, approximately 40 percent are protein-based vaccines, which rarely work without an adjuvant. Yet adjuvants have never attracted much funding from industry and government. “Adjuvants have been the weak link in vaccines for the last hundred years,” says Nikolai Petrovsky, a vaccine researcher at Flinders University in Adelaide, Australia.