There are also some biological reasons why some people feel sick after a breakthrough infection, particularly when they contract the Delta variant. It’s difficult to build up a robust immune response in the nose and throat, particularly when a shot is delivered into the arm. And Delta is so adept at multiplying quickly in upper airway cells that it can start causing symptoms before the immune system kicks into gear.

But the vaccine-conferred immune response is still excellent at fending off the virus before it can invade the lungs, which is when severe disease occurs. A study published Thursday also showed that, in monkeys, lower levels of antibody were needed to halt the virus in the lungs than in the upper airway — suggesting why the vaccines were better at protecting against severe disease than they were at blocking infection entirely.