Tolerability, safety, and preliminary immunogenicity assessment of different dosages of Quadrivalent inactivated influenza vaccine in healthy subjects aged 6-35 months: A single-center, dose-escalation, open-label phase I clinical trial.

This study aims to evaluate the tolerability, safety, and preliminary immunogenicity of quadrivalent influenza virus split vaccine (IIV4) in healthy infants and young children aged 6-35 months across different dosages. Employing a single-center, dose-escalation, open-label Phase I clinical trial design, 60 participants were randomly divided into a low-dose group (0.25 mL per dose) and a high-dose group (0.5 mL per dose). Participants received two intramuscular doses administered 28 days apart. Safety results: The overall adverse reaction rates were 23.3 % in the low-dose group and 20.0 % in the high-dose group, with no statistical difference (P > 0.05). One case of grade 3 local adverse reaction (erythema at the injection site) occurred in the high-dose group, and the remaining were mild reactions of grade 1-2. No severe adverse events were related to the vaccine. Immunogenicity results: Total population: After two doses, the GMT and GMI against H3N2 and B/Yamagata influenza strains in the high-dose group suggested a potential superiority over the low-dose group (P < 0.05). Pre-vaccination susceptible population: The high-dose group exhibited a more significant increase in GMT against H3N2 (31.7 vs. 13.2, P = 0.0482). Additionally, both groups showed significantly higher seroconversion rates, GMT, and GMI after two doses compared to one dose (P < 0.05), confirming that the two-dose schedule effectively enhances immune responses. Notably, the high-dose group demonstrated a more pronounced trend of response enhancement.

Conclusion

Both the low-dose group and high-dose group IIV4 demonstrated good tolerability and safety in infants and young children aged 6-35 months. The high-dose group showed a potential advantage in immune responses to certain antigens. However, due to limitations such as the small sample size and exclusion of high-risk populations, further multi-center, large-sample Phase III clinical trials are needed to validate the clinical significance of the dose effect and provide evidence for influenza prevention and control strategies in young children. Trial registration: CXSL1800038.