Positive Phase II data for GSK's mRNA seasonal influenza vaccine candidate FLUm3HA.b-3NA have cleared the bar for a Phase III efficacy trial, with the UK giant reporting higher immune responses against all tested influenza strains compared to both standard-dose and high-dose licensed inactivated vaccines in younger and older adults. The findings were presented at the OPTIONS XIII Conference for the Control of Influenza in Washington, DC on August 31, 2026.

The data come from Flu-028, a randomised, observer-blind Phase II trial enrolling 971 adults aged 18 and older. Participants aged 18–64 and those aged 65 and above received different dose levels of either FLUm3HA.b-3NA, which encodes an optimised B-strain hemagglutinin (HA), or FLUm3HA-3NA, encoding a non-optimized B-strain HA, alongside age-appropriate licensed comparators. Immunogenicity and reactogenicity were assessed through day 181 post-vaccination. FLUm3HA.b-3NA demonstrated robust immune responses to both HA and NA antigens from influenza A and B strains across age groups, with acceptable reactogenicity and safety profiles, according to GSK. The company said it will carry the optimised B-strain HA formulation forward into Phase III, which is scheduled to begin in September 2026.

The candidate encodes both HA, which mediates viral attachment to host cells, and neuraminidase (NA), which facilitates viral release and spread — a dual-antigen approach. Evidence that NA-directed immunity can contribute to protection against illness severity and transmission exists, though no approved vaccine has yet been designed around it. The US FDA granted the candidate Fast Track designation for its targeted age indication in July 2026.

In the broader mRNA vaccine landscape, GSK is pursuing a dual HA/NA approach. Earlier antibody-based efforts to exploit conserved influenza antigens, including Roche's anti-HA stalk monoclonal antibody MHAA4549A (gedivumab), demonstrated viral load reductions in Phase II challenge models but did not translate to clinical benefit in hospitalized patients. GSK's Phase III trial will be the first to test whether the dual HA/NA mRNA approach is effective.

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