“There is an ongoing reaction in our lymph nodes that’s going for six months. And that reaction, we are showing, that it is actually enhancing the potency of the antibodies,” he said. “Even before the third dose.”

The maturation of the immune response — which would be accelerated by a third dose, given after a longer interval — brings big benefits. The antibodies we make improve in quality through a process called affinity maturation. They become more capable of recognizing their viral target, even if that target has made changes to its appearance, in the way variants have done.

“If you wait six months or more between a priming and boost, what you see is not just a boosting of the immunity that you got from the first vaccine, but you see a broadening of the immune response so that it recognizes other viruses or other variants. Your immune response becomes much more cross-protective,” said David Topham, an immunologist at the University of Rochester Medical Center and director of the New York Influenza Center of Excellence.