Post-Marketing Safety Surveillance of Quadrivalent Influenza Vaccine (VaxigripTetra) in Children Aged 6 to 35 Months in South Korea

Introduction: The quadrivalent inactivated split-virion influenza vaccine (QIV; VaxigripTetra((R))) was initially licensed in South Korea in 2017 for immunization against seasonal influenza in those aged >= 3 years, with the indicated age subsequently lowered to include those aged >= 6 months in 2018. Here, to comply with South Korean licensure requirements, we undertook a post-marketing surveillance study to assess the safety of QIV in children aged 6-35 months (i.e., extension of the previous age indication to include these young children) in routine clinical practice. Methods: A multicenter, observational, active safety surveillance of children aged 6-35 months who received a single dose of QIV during a routine healthcare visit was undertaken in South Korea from 15 June 2018 to 14 June 2022. Solicited adverse events (AEs) and unsolicited non-serious AEs were recorded in diary cards, with serious adverse events (SAEs) notified to study investigators. Results: This safety analysis included 676 participants. No AEs led to study termination, and no SAEs were reported. The most frequent solicited injection site reaction was pain in both the <= 23-month (12.2% [55/450]) and >= 24-month (15.5% [35/226]) age groups. The most frequent solicited systemic reactions were pyrexia and somnolence in the <= 23-month age group (6.0% [27/450] each), and malaise (10.6% [24/226]) in the >= 24-month age group. Overall, 208 (30.8%) participants experienced 339 unsolicited non-serious AEs, with nasopharyngitis the most common (14.1% [95/676]), and nearly all events (98.8% [335/339]) were considered unrelated to QIV. Grade 3 solicited reactions and unsolicited non-serious AEs were reported in five (0.7%) and three (0.4%) participants, respectively, all of whom recovered by day 7 after vaccination. Conclusion: This active safety surveillance study confirms that QIV is well tolerated in children aged 6-35 months in routine clinical practice in South Korea. There were no safety concerns observed in these young children.