SINOVAC's Quadrivalent Influenza Vaccine Demonstrates Non-inferior Immunogenicity to an International Comparator, with Cellular Immune Mechanisms Underlying Durable Protection

BEIJING, Dec. 16, 2025 – Results from a Phase III clinical study evaluating both humoral and cellular immune responses induced by SINOVAC's quadrivalent influenza vaccine (Sinovac-QIV) have been published in Nature Communications. The study further confirmed the vaccine's immunogenicity and safety profile, and for the first time revealed the cellular immune basis for its strong and durable immune response.

The published findings are based on a subgroup analysis from an international multicenter Phase III clinical trial conducted in Chile and the Philippines in 2023. The subgroup included 263 adults aged 18 to 64 years enrolled in Chile, including individuals with chronic comorbidities. Of these, 132 volunteer subjects received SINOVAC-QIV and 131 received a licensed international comparator (control group).

Comparable Immunogenicity and Safety Profile

Humoral immune response results at 28 days post-vaccination showed that SINOVAC-QIV induced a 9- to 10-fold increase in geometric mean titers (GMTs) against the A/H1N1, A/H3N2, and B/Victoria strains from baseline, compared with a 7- to 8-fold increase in the comparator group. For the A/H1N1 strain, the post-vaccination GMT in the SINOVAC-QIV group was 1.4 times that of the comparator group.

For the A/H1N1 and A/H3N2 strains, seroconversion rates in the SINOVAC-QIV group at 28 days post-vaccination were significantly higher than those in the comparator group (81.1% vs. 60.3% and 98.5% vs. 69.5%, respectively; p < 0.05). The seroprotection rate for A/H1N1 reached 87.9% in the SINOVAC-QIV group, significantly higher than 77.1% in the comparator group. Seroprotection rates reached 98.5% for A/H3N2 and 100% for both B/Victoria and B/Yamagata.

In terms of safety, the study showed confirmed that the safety profile of SINOVAC-QIV was comparable to that of the international comparator vaccine. Most adverse events reported after vaccination were mild to moderate, with commonly reported events including injection-site pain, headache, and fatigue. No vaccine-related serious adverse events were reported throughout the study period.

Cellular Immune Mechanisms Underlying Durable Immune Response

The study employed ELISPOT, multiparameter flow cytometry, Furthermore, it innovatively applied unsupervised high-dimensional data reduction (bh-SNE) and clustering analysis (Phenograph) to characterize vaccine-induced cellular immune responses.

The findings showed that cellular immune responses induced by SINOVAC-QIV were associated with reduced T-cell exhaustion phenotypes and increased follicular helper T-cell phenotypes. These features are considered relevant to long-term immune memory and provide a cellular immune basis for the durability of the vaccine-induced immune response.

Global Public Health Impact

SINOVAC's seasonal influenza vaccines have been supplied to nearly 20 countries and regions, with cumulative use exceeding 100 million doses. In 2025, global shipments exceeded 20 million doses, including supply for Chile’s publicly funded influenza vaccination program. This study in Chile demonstrates that inactivated influenza vaccines can synergistically stimulate both humoral and cellular immune responses. These findings provide new insights into the protective mechanisms of influenza vaccines and offer an evidence-based foundation for the continued global application of inactivated vaccine technology.

Reference:

The full research paper, titled "Influenza vaccines promote humoral and cellular immune responses: a randomized, double-blind, phase 3 trial", is available at: https://doi.org/10.1038/s41467-025-67102-y.

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