Eventually, all discussions about sterilizing immunity [Ability to totally prevent infection] become nerdy quibbles over semantics. Clearly, not every infection is clinically meaningful, or even logistically detectable, given the limits of our technology—nor do they need to be, if there’s no sickness or transmission. (A koan for pandemic times: If a microbe silently and inconsequentially copies itself in a tissue, and the body doesn’t notice, did it actually infect?) There is, for every pathogen, a threshold at which an infection becomes problematic; all the immune system has to do is suppress its rise below this line to keep someone safe.
But that might be exactly the point. Say that sterilizing immunity is impossible, that our immune systems cannot, in fact, be trained to achieve perfection. Then it’s neither a surprise nor a shortcoming that COVID-19 vaccines, or other vaccines, don’t manage it: An inoculation that guards marvelously well against disease—offering as much protection as it can—can still end an outbreak. Life would certainly be easier if vaccines offered invincible armor, with pathogens simply ricocheting off. But they don’t, and assuming or expecting them to manage that can be dangerous. The dubiousness of sterilizing immunity is a reminder that just about any immune response can be overwhelmed, if exposures are heavy and frequent enough, Grad told me. The best we can all hope for is functional immunity, more like a flame retardant than a firewall, that still keeps bad burns at bay.