Modern manifestations of the epidemic process of measles infection in St. Petersburg and the Leningrad region

Objective: To identify the features of the epidemic process of measles in Saint Petersburg and the Leningrad region during the period of increased morbidity. Materials and Methods: A retrospective epidemiological analysis based on data from the accounting and registration database of infectious and parasitic diseases of the Federal Budgetary Institution «Center for Hygiene and Epidemiology in Saint Petersburg and the Leningrad Region» for the period from January 1, 2023, to August 1, 2024. Results: A total of 1,044 confirmed cases of measles were recorded, with an incidence rate of 13,80 (95% CI 11.71–15.89) per 100,000 population (13,31 in St. Petersburg and 13,41 in the Leningrad region). The incidence among children was 6,95 times higher thanthatamong adults (p<<0,01): 8,57 times in the Leningrad region and 6,84 in St. Petersburg. The highest incidence was observed among children aged 1–2 years (110,82 per 100,000, 95% CI from 3,95 to 12,1 per 100,000). In 2024, there was a sharp increase in incidence from January to April. In the age structure, children under one year old accounted for 1.08%, 1–2 years – 14,08%, 3–6 years – 13,10%, 7–14 years – 24,54%, 15–17 years – 5,28%, 18–35 years – 18,57%, 36–55 years – 20,04%, and 55 years and older – 3,32%. Unorganized children made up 25%, organized children 13%, school students 22%, university students 2%, working adults 23%, and non-working adults 15%. A late appeal for medical assistance was identified among the sick (50.9% on the fourth day or later from the onset of symptoms). Contact with a measles-infected individual was observed in 34.10% of cases: 41,82% among children and 23,46% among adults. 82,47% of those infected were unvaccinated, 11,87% were vaccinated according to the schedule, 4,31% were vaccinated for epidemiological reasons, 1,15% received measles immunoglobulin, and 0.19% had previously had measles. Conclusion: During the activation of the epidemic process, manifestations of the epidemic process take on characteristics typical of the pre-vaccination period, namely: pronounced winter-spring seasonality, high incidence among young children, including those under one year old, and significant focality. The high level of morbidity is sustained by unvaccinated individuals, and the late appeal for medical assistance contributes to the spread of measles.