In several prior articles I have attempted to analyze and simplify the subject of immunological functions induced by vaccination that correlate with protection against later exposure to pathogens. Other authors have also written on the subject, and recently we jointly proposed terminology to bring some semantic clarity to the field. The generalization that vaccine-induced antibodies prevent acquisition whereas cellular immune functions clear infection still holds true, but that simple distinction becomes blurred in many instances. Specific antibody and cellular responses are multiple and redundant, so that vaccines for some pathogens protect through more than 1 immune function. Thus, this article aims in the direction opposite to simplicity to depict the complexity of correlates, or rather the complexity of mechanistic immune functions that contribute to protection. Nonmechanistic correlates that are practically useful but not truly protective will be mentioned in passing.