Clinical and epidemiological characteristics of an influenza А (H3N2) outbreak in an organized group of children

Background. Influenza outbreaks in closed pediatric residential settings represent a significant epidemiological challenge due to the high contagiousness of the pathogen and close-contact living conditions. Of particular interest are cases of infection among vaccinated children, which permit the assessment of the modifying effect of vaccination on the course of the disease.

Objective. To investigate the clinical, epidemiological, and laboratory characteristics of an A (H3N2) influenza outbreak in a closed-type child care institution.

Methods. A cohort study was conducted involving 25 cases of acute respiratory infection in children and adolescents aged 4–16 years residing in a closed-type social welfare institution (Sverdlovsk Oblast, Russia) from November 18 to November 28, 2025. Influenza A (H3N2) was confirmed by PCR in all hospitalized patients.

Results. The outbreak exhibited a classic point-source epidemic curve, peaking on November 22, 2025 (17 cases). All hospitalized patients (n = 19) tested positive for influenza A(H3N2). Unvaccinated children demonstrated higher rates of gastrointestinal symptoms (abdominal pain 57 vs. 0 %), higher CRP levels (35.2 vs. 24.5 mg/L), and 100 % hospitalization rate compared to 67 % in vaccinated children. Oseltamivir was administered to all hospitalized patients.

Conclusion. Vaccination did not prevent infection, while significantly modifying the disease severity, reducing hospitalization rates and clinical-laboratory manifestations.

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