Much of the focus has been on levels of antibodies, which serve as one of the immune system’s front-line defenses. One theory about Moderna’s vaccine is that it creates more of those antibodies because it uses a larger dose and the two doses are administered over a one-week longer period than Pfizer’s.

But antibodies are just one component of immunity, and it isn’t clear if they are the most important one, especially over the long-term.

“Do we know an antibody level that protects against Covid? The simple answer is we still do not know that,” said Paul Burton, Moderna’s chief medical officer, in a Friday call with reporters. Still, Moderna’s trial data show that a third shot six months after the second raises antibody levels “well into that comfort zone” back above levels seen in the initial phase 3 trial.