Specific measles, mumps, and rubella prevention has been the main prerequisite for a striking decline in the incidence of such infections in Russia. An increase in the percentage of seronegative individuals observed in recent years resulted in higher measles incidence being directly related to low herd immunity that accounts for a population protection solely under conditions of a high density of immunized individuals and their uniform distribution in the population. The number of immunized individuals may be estimated only while conducting seroepidemiological monitoring of herd immunity. Objective of the study: to assess a level of herd immunity in the St. Petersburg and Leningrad Region population against measles, mumps, and rubella viruses. Materials and methods. There were enrolled 6,774 residents into the study: volunteers aged from 1 to 70+ years. The representativeness of the surveyed cohort was ensured by using the Web application "Monitoring of herd immunity against socially significant infections", used at the stage of volunteer enrollment, by randomization and regulation of the sample size in age groups. Participants filled out a questionnaire and agreed to provide venous blood samples to assess IgG antibody levels against measles, mumps, and rubella viruses by using ELISA. Results. In September 2023, in St. Petersburg and the Leningrad Region, herd immunity met the criterion for epidemiological well-being only with respect to rubella. In all age groups, the proportion of seronegative individuals did not exceed 15%, and most volunteers had high Ab levels, both after illness and vaccination. For measles and mumps, the criterion for epidemiological well-being is considered not to exceed more than 7% seronegative individuals. A sufficient level of measles seroprevalence was detected only in older age groups (≥60 years old). Sufficient mumps seroprevalence was not detected in any age group. The average population (St. Petersburg, Leningrad Region) seroprevalence magnitude for measles, rubella, and mumps viruses were 81.4%, 95.5%, and 78.4%, respectively. The problematic age groups with low measles seroprevalence (62.4–74.3%) were adolescents (12-17 yrs) and young adults (18-39 yrs). Most seropositive individuals vaccinated against measles had low Ab levels; high levels were noted mainly in older measles convalescent individuals. Low mumps seroprevalence (~70%) was more often observed among adults aged 18 to 49 years. The distribution of seroprevalence in various occupational group was relatively uniform, with some predominance of seropositivity among pensioners and schoolchildren. Conclusion. The system of specific prophylaxis for vaccine-preventable viral infections used in Russia has shown high efficacy and contributed to the formation of herd immunity, which for many years allowed to lower a risk of both sporadic and group infections to minimal levels. Currently, measles and mumps seroprevalence in the local population is maintained at insufficient level to ensure epidemiological well-being. This necessitates making appropriate management decisions and conducting additional preventive measures aimed at enhancing relevant herd immunity.
