When breakthrough cases do arise, it’s not always clear why. The trio of vaccines now circulating in the United States were all designed around the original coronavirus variant, and seem to be a bit less effective against some newer versions of the virus. These troublesome variants have yet to render any of our current vaccines obsolete. But “the more variants there are, the more concern you have for breakthrough cases,” Saad Omer, a vaccine expert at Yale, told me. The circumstances of exposure to any version of the coronavirus will also make a difference. If vaccinated people are spending time with groups of unvaccinated people in places where the virus is running rampant, that still raises their chance of getting sick. Large doses of the virus can overwhelm the sturdiest of immune defenses, if given the chance.

The human side of the equation matters, too. Immunity is not a monolith, and the degree of defense roused by an infection or a vaccine will differ from person to person, even identical twins. Some people might have underlying conditions that hamstring their immune system’s response to vaccination; others might simply, by chance, churn out fewer or less potent antibodies and T cells that can nip a coronavirus infection in the bud.