For COVID-19 vaccines, booster is already a popular term, but it’s not obvious how restorative the additional shots are, in terms of guarding against the coronavirus. In one group, at least, third shots are generative: people who are moderately or severely immunocompromised, and may not have marshaled a sufficient immune response to their initial vaccine doses. “In this population, that’s really clear,” Grace Lee, a pediatrician at Stanford University and the chair of the CDC’s Advisory Committee on Immunization Practices, told me. (There is still, frustratingly, a huge paucity of data on the one-dose Johnson & Johnson vaccine, though several experts have told me in recent weeks that J&J’s regimen may become a two-shot primary series for everyone, based on the company’s recent findings.)

When it comes to the rest of us, especially people who are younger and healthier, experts remain divided on how to categorize third shots. Anthony Fauci told me recently that he’s very much in the generative camp: “I bet you any amount of whatever that when we finally look back on it,” he said, three doses is going to be “the standard regimen for an mRNA vaccine.” (Still, even Fauci’s been blurring the semantic boundaries. In a recent interview with my colleague Ed Yong at The Atlantic Festival, he alternately described the shot as a “third dose,” a “third-shot booster,” and a “third booster shot” in a five-minute span.)

If that turns out to be the case, experts would first need to show that what the first two doses gave us wasn’t good enough, opening up the opportunity for a third jab to make our defenses “more durable, and much more able to protect us” than they were with two shots alone, Paul Offit, a vaccine expert at the Children’s Hospital of Philadelphia, told me. But so far, there’s really no clear evidence to suggest that a third shot elevates us into a new tier of protection, especially against the worst COVID-19 outcomes.