Effectiveness of the Cell-Based Quadrivalent Influenza Vaccine (SKYCellflu^® QIV) in Children and Adolescents: A Multicenter Test-Negative Case–Control Study in Korea

Background: Children and adolescents are pivotal in the transmission of influenza, and vaccination remains the most effective preventive measure. Cell-based influenza vaccines offer advantages over traditional egg-based vaccines by reducing egg-adapted mutations and improving antigenic match. SKYCellflu^® quadrivalent influenza vaccine (QIV; SK bioscience, Korea), the first cell-based QIV licensed in Korea for individuals aged 6 months and older, offers potential advantages; however, its real-world effectiveness in the Korean pediatric population remains limited. Objective: This study aimed to estimate the real-world effectiveness of SKYCellflu^® QIV, a cell-based QIV, in preventing laboratory-confirmed influenza among children and adolescents aged 6 months to 18 years in Korea during the 2024–2025 influenza season. Methods: A multicenter, prospective, test-negative case–control study was conducted from October 2024 to May 2025 across 25 institutions in Korea. Children and adolescents aged 6 months to 18 years who presented within 7 days of the onset of influenza-like illness (fever ≥ 38 °C and at least one respiratory symptom) were enrolled. Influenza infection was confirmed using rapid antigen tests or polymerase chain reaction; participants who tested positive were classified as cases, and those who tested negative for influenza served as controls. All participants were further categorized as vaccinated or unvaccinated based on receipt of SKYCellflu^® QIV. Those who received other influenza vaccines during the season were excluded. Vaccination status was verified through medical records and the national immunization registry. Results: A total of 1476 participants were included (751 cases, 725 controls). The overall adjusted vaccine effectiveness (aVE) was 45.57% (95% CI, 29.38–58.04). The vaccine demonstrated the highest effectiveness in children aged 6–35 months (aVE: 88.55%; 95% CI, 60.39–96.11). Effectiveness was higher against influenza B (aVE: 61.28%; 95% CI, 35.76–76.30) than influenza A (aVE: 41.63%; 95% CI, 22.55–56.01). The vaccine’s effectiveness in adolescents was not statistically significant due to the small sample size in this age group. Conclusions: This multicenter test-negative study provides the first real-world effectiveness of SKYCellflu^® QIV in a Korean pediatric population. The results suggest substantial protection in younger children, particularly against influenza B, and support the continued use of annual influenza vaccination in this population. Further studies with larger adolescent cohorts are needed to confirm these findings in older age groups.