COVID-19 Vaccine Success Could Be Measured With One Number

The term correlate of protection doesn’t exactly roll off the tongue, but it’s one of the sexiest concepts in the field of vaccinology. Correlates are biological benchmarks—measurements of a single immune molecule or cell—that can show that a vaccine is achieving its desired effect. With a correlate in hand, researchers can confirm how well a shot is working and identify the rare individuals in whom it doesn’t take; they can suss out the need for boosters and fast-track the development of new vaccines. At their most powerful, correlates of protection boil down the complexities of an immune response to a single value—one that can confidently affirm that a person won’t get infected or seriously sick. “It’s kind of a magic number,” Ali Ellebedy, an immunologist at Washington University in St. Louis, told me. “It’s the big holy grail,” Emory University’s Sri Edupuganti says. “It’s what we dream about,” Cornell’s Sallie Permar told me last month.