LONDON, UK, Sept. 1, 2026
GSK announced that it will advance its investigational mRNA seasonal influenza vaccine candidate into Phase III development after positive results from the Phase II Flu-028 trial. The company reported that the vaccine generated higher immune responses than licensed standard-dose and high-dose inactivated flu vaccines in younger and older adults, respectively, across all influenza strains evaluated. Based on these findings, GSK plans to initiate a Phase III efficacy trial in September 2026. The candidate remains investigational and is not approved anywhere in the world.
GSK Reports Positive Phase II Vaccine Results
The Phase II Flu-028 study was a randomized, observer-blind trial evaluating the immunogenicity and safety of mRNA-based multivalent seasonal influenza vaccine candidates in 971 adults aged 18 years and older. Participants included adults between 18 and 64 years of age and adults aged 65 years and older, with different vaccine dose levels evaluated alongside licensed age-appropriate influenza vaccine comparators. Immune responses and safety outcomes were assessed through Day 181 following vaccination. GSK reported that its optimized candidate, FLUm3HA.b-3NA, produced robust immune responses against hemagglutinin (HA) and neuraminidase (NA) from influenza A and B strains in both younger and older adults. The vaccine candidates were also described as generally well tolerated, with acceptable reactogenicity and safety profiles observed during the study. These findings provided the basis for GSK’s decision to move the optimized candidate into late-stage clinical development. The results are significant because seasonal influenza continues to represent a substantial global health burden, particularly among older adults, pregnant women and people with underlying health conditions. GSK estimates that seasonal influenza causes around one billion cases globally each year, including millions of severe cases and hundreds of thousands of respiratory deaths.
mRNA Candidate Targets HA and NA Antigens
GSK’s vaccine represents a differentiated approach to influenza vaccination because it is designed to target two major viral surface antigens: HA and NA. Conventional licensed influenza vaccines primarily focus on hemagglutinin, which plays a central role in enabling influenza viruses to attach to and enter host cells. GSK’s investigational mRNA approach additionally targets neuraminidase, which has an important role in viral release and spread. The company believes targeting both antigens could potentially contribute to broader or improved protection against influenza, including effects on illness severity and viral transmission. The Phase II results showed immune responses to both HA and NA, providing clinical support for continued evaluation of the dual-antigen strategy. However, the Phase II findings primarily provide evidence of immunogenicity and safety; whether the approach delivers superior clinical protection against influenza illness will need to be determined in the planned Phase III efficacy trial. GSK said the upcoming Phase III study will represent the first Phase III trial of an mRNA influenza vaccine specifically designed to target both HA and NA. The trial is expected to provide more definitive evidence regarding the candidate’s ability to prevent influenza and support future regulatory evaluation if the results are positive.
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GSK Expands Next-Generation Flu Vaccine Program
The advancement of the candidate adds momentum to GSK’s mRNA vaccine research program and broader efforts to develop next-generation respiratory vaccines. In July 2026, the U.S. Food and Drug Administration granted the vaccine candidate Fast Track designation for the targeted age indication, reflecting the potential importance of improved seasonal influenza protection. GSK said the Phase II findings represent a significant advancement for its mRNA program and support the company’s objective of developing a potentially improved influenza vaccine. The Phase III program will now become an important test of whether the strong immune responses observed in Phase II translate into meaningful protection against seasonal influenza. The candidate’s progression also demonstrates the expanding application of mRNA technology beyond COVID-19 vaccines, with pharmaceutical developers investigating the platform for seasonal influenza and other infectious diseases. For GSK, the upcoming Phase III trial will provide critical clinical evidence as the company seeks to determine whether its dual HA/NA approach can improve protection against a disease that continues to cause significant global morbidity and mortality.
Source: GSK press release
