Consider, first, what happens when a vaccine trains a body using a near-perfect pantomime of the pathogen that later appears. A COVID-19 shot pumps in a little lesson on coronavirus spike, modeled on the original virus; immune cells study its contents and panic, then scramble to sweep away the interloper. When the actual virus appears, the process repeats itself more swiftly and smoothly. T cells home in on infected cells and annihilate them; antibodies, churned out by B cells, anchor themselves stubbornly all over the spike, gumming on as tightly as “superglue,” as Christopher O. Barnes, a structural biologist and antibody expert at Stanford, puts it. This sticky strategy is particularly powerful: Antibodies can prevent SARS-CoV-2 from using its spike to dock onto vulnerable cells, or earmark the virus for violent destruction. The microbe can be cleared from the body before it even has time to cause symptoms or spread to someone else.

When a new version of the virus shows up, freckled with mutations, certain antibodies may start to lose their grip. (More than 30 of Omicron’s mutations are in its spike.) Some could stop tethering to the microbe entirely, while others might slip on and off the pathogen as if slicked with heavy palm sweat. That leaves the virus’s key protein uncovered more frequently, giving the microbe “more opportunity to interact with your cells,” Goel said, and wriggle its way inside.

That scenario is less than ideal but not necessarily a crisis. Spike’s a big protein, and some of the antibodies sparked by the original vaccines should still be stage-four clingers. Even antibodies with subpar stickiness “can still act in concert,” as long as they’re abundant enough, Marion Pepper, an immunologist at the University of Washington, told me.