She was able to confirm that in June through a simple test that searched her blood for antibodies that recognize the rubella virus, and then added them up. If her antibody counts were above a certain level, called a correlate of protection, she and her babies would be considered well shielded from disease. “You are considered immune with a titer of 9.9 to rubella,” she tweeted last month, referring to her antibody levels. “My titer? 116. I love my immune system sometimes.”

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.

In recent weeks, the correlate community has been buzzing louder than ever. Scientists are on the cusp of confidently defining some correlates of protection against symptomatic disease for the COVID-19 vaccines. If confirmed, these correlates could revolutionize the way we tackle SARS-CoV-2 immunization: Vaccine makers testing a new inoculation may no longer need to follow tens of thousands of people for many months to test their product’s protection. Instead, they could inject just a few hundred people, snag some drops of blood, and see if the elusive correlate is met. That’s how we tee up new flu vaccines every year without the rigmarole of gargantuan clinical trials.