To understand the anatomy of a breakthrough case, it’s helpful to think of the human body as a castle. Deepta Bhattacharya, an immunologist at the University of Arizona, compares immunization to reinforcing such a stronghold against assault.

Without vaccination, the castle’s defenders have no idea an attack is coming. They might have stationed a few aggressive guard dogs outside, but these mutts aren’t terribly discerning: They’re the system’s innate defenders, fast-acting and brutal, but short-lived and woefully imprecise. They’ll sink their teeth into anything they don’t recognize, and are easily duped by stealthier invaders. If only quarrelsome canines stand between the virus and the castle’s treasures, that’s a pretty flimsy first line of defense. But it’s essentially the situation that many uninoculated people are in. Other fighters, who operate with more precision and punch—the body’s adaptive cells—will eventually be roused. Without prior warning, though, they’ll come out in full force only after a weeks-long delay, by which time the virus may have run roughshod over everything it can. At that point, the fight may, quite literally, be at a fever pitch, fueling worsening symptoms.

Vaccination completely rewrites the beginning, middle, and end of this story. COVID-19 shots act as confidential informants, who pass around intel on the pathogen within the castle walls. With that info, defensive cells can patrol the building’s borders, keeping an eye out for a now-familiar foe. When the virus attempts to force its way in, it will hit “backup layer after backup layer” of defense, Bhattacharya told me.