Preliminary Safety Assessment of a Quadrivalent Subunit Influenza Vaccine in China: A Phase IV Study.

INTRODUCTION: The aim of this work is to preliminarily evaluate the post-marketing safety of a quadrivalent subunit influenza vaccine in healthy individuals aged 3 years and above through active surveillance of adverse reactions following vaccination. METHODS: During the 2024-2025 influenza season, healthy participants aged 3 years and older were enrolled in Jiangsu Province. All participants received a single dose of the quadrivalent subunit influenza vaccine. Adverse events (AEs) and serious adverse events (SAEs) were monitored within 0-28 days post-vaccination, and information on SAEs occurring within 6 months post-vaccination was also collected. Descriptive analyses were performed to calculate the reporting rates of AEs, adverse drug reactions (ADRs) and SAEs. We compared differences in overall and symptom-specific ADR reporting rates across age groups and sexes. RESULTS: A total of 3105 participants completed vaccination and were included in the safety analysis set. A total of 282 participants reported 477 AEs, yielding an overall AE reporting rate of 9.08%. A total of 188 participants reported 318 ADRs, with an ADR reporting rate of 6.05%. The most frequently reported ADR symptoms were tenderness (69 episodes, reporting rate 2.22%), cough (57 episodes, 1.83%), and fever (46 episodes, 1.48%). The majority of ADRs (272 episodes, 85.53%) were grade 1 in severity, and most symptoms (253 episodes, 79.56%) resolved within 3 days of onset. No vaccine-related SAEs were reported, and no new serious adverse reactions were identified. CONCLUSIONS: This active surveillance study demonstrated a favorable safety profile for the quadrivalent subunit influenza vaccine, with low ADR reporting rates and predominantly mild, transient reactions. These findings provide reliable evidence supporting the preliminary post-marketing safety assessment of the vaccine in China.