This review by Stanley A. Plotkin discusses the correlates of protection induced by vaccines, focusing on the immune responses that correlate with protection. The author emphasizes that while antibodies are crucial, they are not the sole factor, and other immune functions such as cellular immunity and memory play significant roles. The article covers various types of vaccines, including those against encapsulated bacteria, toxin-producing bacteria, bacteria introduced directly into the bloodstream, intracellular bacteria, viruses transmitted by arthropods, viruses infecting mucosae and causing viremia, hepatitis viruses, and viruses replicating only on mucosae. It highlights the importance of specific antibody levels, the role of cellular responses, and the significance of memory in providing long-term protection. The review also addresses the complexities in defining correlates for some vaccines, such as those against influenza and rotavirus, and concludes by emphasizing the multifaceted nature of immune responses in vaccine-induced protection.This review by Stanley A. Plotkin discusses the correlates of protection induced by vaccines, focusing on the immune responses that correlate with protection. The author emphasizes that while antibodies are crucial, they are not the sole factor, and other immune functions such as cellular immunity and memory play significant roles. The article covers various types of vaccines, including those against encapsulated bacteria, toxin-producing bacteria, bacteria introduced directly into the bloodstream, intracellular bacteria, viruses transmitted by arthropods, viruses infecting mucosae and causing viremia, hepatitis viruses, and viruses replicating only on mucosae. It highlights the importance of specific antibody levels, the role of cellular responses, and the significance of memory in providing long-term protection. The review also addresses the complexities in defining correlates for some vaccines, such as those against influenza and rotavirus, and concludes by emphasizing the multifaceted nature of immune responses in vaccine-induced protection.