When and if the maxim’s second part—a rebound in vaccination—might manifest “is the key question,” Paul Offit, a pediatrician and vaccine expert at Children’s Hospital of Philadelphia, told me. Experts anticipate a shift eventually. Vaccine coverage has often been beholden to a kind of homeostatic pull, in which it dips and then ricochets in response to death and suffering. In 2022, for instance, in the weeks after polio paralyzed an unvaccinated man in Rockland County, New York, the families of more than 1,000 under-vaccinated children heeded advice to immunize.