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(®)) 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.