Just as important as sussing out our need for shots is determining how many our immune systems (and psyches) can handle. At a certain point, yet another exposure to the exact same vaccine just won’t do the body’s defenses much good. Our current vaccination regimens aren’t running this risk yet. But repeatedly dosing every few months may rack up unnecessary costs.

Some are logistical. The more vaccines we need, the more we’ll have to manufacture, and the more often public-health officials will have to convince communities to accept them. Side effects can keep people out of school or work, and researchers don’t yet know to what extent boosting might raise the risk of rare, serious events such as heart inflammation. Faced with an unending series of shots, some people might stop getting them, or never start the vaccine series at all. Cumbersome dosing regimens could also exacerbate vaccine inequities, as countries with fewer resources struggle to administer repeat shots.

There’s good reason to wait between doses, too. A stretched-out interval can give antibodies more time to mature. Ellebedy’s team, which has been tracking this prolonged antibody coming-of-age, has found that, half a year out from the second mRNA dose, many molecules are still on their self-improvement kick. Waiting at least a few months could help ensure that the mediocre antibodies get weeded out, leaving only the best to be called into action.