Immunological memory is one of the key features of the adaptive immunity. It confers the ability to resist infection and prevents development of cancer or autoimmune diseases. Most importantly, immunological memory mediated by preexisting antigen- specific clones of T- and B-cells ensures a rapid and effective response to an invasion by a previously encountered pathogen. Since vaccination induces a specific long-lived response to infectious agents, it becomes a basis for preventive medicine. In a human population, immunological memory of individuals shapes the so-called herd or community immunity crucial for national health. The present review touches upon significant population-wide research studies of immunological memory with regard to immunization. We discuss the principles of serological testing and the outcomes of serological monitoring conducted in different countries, and talk about standard and innovative analytical approaches to studying immunological memory. We also pinpoint the drawbacks of methods used for herd immunity assessment in Russia and propose a comprehensive system for seroepidemiological surveillance.