Long-term dynamics and age structure of the incidence of Shigella sonnei and Shigella flexneri infections in the Russian Federation, 2009–2024

Aim. To analyze long-term trends and age-specific patterns of S. sonnei and S. flexneri incidence in the Russian Federation over the period 2009–2024.

Materials and methods. A retrospective analysis was conducted using data from the federal statistical surveillance (Form No. 2 «Information on infectious and parasitic diseases») for 2009–2024. Incidence rates were calculated per 100,000 population. Average annual incidence rates (AAIR) were computed as arithmetic means of annual values. The average annual rate of decline (increase) in incidence (AAR) was estimated from the linear regression coefficient to assess long-term trends. Statistical significance of differences was evaluated using Student's t-test, and confidence intervals were calculated by the Wald method.

Results. The incidence of both shigellosis forms continued to decline, but the pace slowed down: for S. flexneri, AAR changed from -25.8 % (2003–2009) to -12.6 % (2009–2024); for S. sonnei, from -17.4 % to -11.6 %. Since 2022, the incidence of S. sonnei has for the first time significantly exceeded that of S. flexneri (previously S. flexneri had dominated for many years). Over the study period, the epidemic process of S. flexneri was characterized by a lack of pronounced cyclicity and a persistent shift of the highest incidence towards the youngest age groups (children under 1 year and 1–2 years; AAIR 14.11 and 17.69, respectively), indicating a predominantly contact-household transmission route. In contrast, S. sonnei showed regular periodic rises every 2–4 years, with peak incidence in children aged 3–6 years (AAIR 10.38); the proportion of cases among children aged 7–14 years was high (22.1 %).

Discussion. The identified shift in the ratio of pathogens (predominance of S. sonnei since 2022) is consistent with international data and may be considered a marker of improved sanitary and hygienic conditions. The decline in incidence in 2020–2021 is attributed to anti-epidemic measures implemented during the COVID-19 pandemic. The parallel dynamics of S. flexneri incidence among children under 2 years of age and adults indirectly indicates a predominantly familial (contact-household) mode of transmission under current conditions of centralized water supply, whereas the distinct dynamics observed among children aged 3–6 and 7–14 years point to the role of organized groups in the spread of S. sonnei.

Conclusion. Divergent changes were identified in the age structure and incidence ratio of S. sonnei and S. flexneri shigellosis, particularly pronounced in the post-pandemic period (since 2022).

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