Fig. 4. A schematic illustrating the role of extant circulating and possibly mucosal antibodies in vaccine induced protection.

Four identical exposure scenarios are depicted. At a higher neutralization titer e.g., 1000 IU50/ml, extant antibody alone results in protection with no need for engagement of B cells, T cells, or other vaccine induced elements. At a lower titer e.g., <100 IU50/ml, mAb antibody alone is not enough for high protection. Thus, vaccine induced protection for lower titers requires engagement of some combination of anamnestic B cell responses (e.g., nAbs antibodies that mediate FcR effector functions), CD8 + T cells, and other vaccine induced immune responses.