A misconception exists that vaccines transform us into totally immune beings, able to instantaneously smite any virus we encounter. Sten H. Vermund, an infectious-disease epidemiologist and dean of the Yale School of Public Health, likened this false idea to a bug zapper: “As soon as it touches my mucosa or skin or genital tract — boom, zap, it’s gone!” Not quite. Instead, Vermund said, vaccines are more similar to poisoned traps, into which a pest might fall, wriggle a bit, then perish from insecticide.