Fauci: We did not always know that a third dose would likely be an important part of the proper, complete regimen. In the early trials, we started off with a prime followed by a booster. And the results were so strikingly good, both in the animal model and in the Phase 1, Phase 2 trials, that we didn’t have the luxury to say, “Wait a minute, we’re going to try multiple different doses and make sure we get it just right.” We were in an emergency situation, so we went with what was really good. Already the track record has shown you that it has saved millions of lives.

What we did not fully appreciate—and there was no way of knowing it—is what the durability of that would be. I’m an immunologist, and I know what other immunologists say is true: The antibody level going down doesn’t necessarily mean that you’re not protected, because you have memory B cells; you have T cells. But the clinical phenomenon is the thing that you need to pay attention to. And that’s what we were seeing: Over time, protection against hospitalization was going down, starting off with the elderly and then even occurring across age groups. Without a doubt, the protection against clinically recognizable disease was going down.

We did not know that during the clinical trials. What we’re starting to see now and fully appreciate is that it is likely, for a real, complete regimen, that you would need at least a third dose. Now, the question is going to be, does that mean we have to boost people every single year? We don’t know that now. What I hope is going to happen is that this is going to get to the human immune system, in vaccinated people, that degree of maturation of response, that will really give much more durability to it.