The classic tale of sterilizing immunity unfolds something like this: A pathogen attempts to infiltrate a body; antibodies, lurking in the vicinity thanks to vaccination or a previous infection, instantly zap it out of existence, so speedily that the microbe can’t even reproduce. No symptoms manifest, and most of the body’s immune cells never get involved, a bit like an intruder smacking up against an electric fence around a building, leaving the security guards inside none the wiser.

This is a very neat story. And it is “almost impossible to prove,” Mark Slifka, an immunologist and vaccine expert at Oregon Health & Science University, told me. To show sterilizing immunity, researchers have to demonstrate that an infection never occurred—a big ask, considering that microbiologists can’t even agree on what an infection actually is. An onslaught of pathogens ravaging the airway or gut certainly counts. But according to some experts, so does a single viral particle commencing the process of copying itself inside a cell. This is further muddled by the fact that many pathogens, including SARS-CoV-2, can set up shop inside their hosts without causing a single symptom. There is, and always has been, a disconnect between infection and disease…

With measles, for instance, scientists initially lacked the tests needed to show them otherwise, Diane Griffin, an immunologist at Johns Hopkins University, told me. When virtually no one fell ill after an inoculation campaign, researchers figured that infections had evaporated as well. Now, however, techniques are far more powerful, giving researchers the ability to zero in on even tiny blips of infection. Post-vaccination measles infections, though still uncommon, are much more “regularly observed” than they were once believed to be, Griffin said.