The Importance of Multiple Doses in SARS-CoV-2 Antigens

It seems like there is some remembrance of antigens passed without help from T-cells. Can you explain from a viewpoint of a germinal center then why, for some antigens, you need to give multiple doses? What is the reason for that? I always thought of the first dose as the one that makes the germinal centers and the affinity maturation,. The other doses are really what they are boosters. They take memory and turn memory into plasma cells.