GSK Advances mRNA Flu Vaccine Candidate to Phase 3 After Positive Phase 2 Data

GSK presented positive phase 2 results for its investigational mRNA seasonal influenza vaccine candidate at the OPTIONS XIII Conference for the Control of Influenza, reporting higher immune responses against all tested influenza strains compared with licensed standard-dose and high-dose inactivated flu vaccines.¹ The randomized, observer-blind Flu-028 trial enrolled 971 adults aged 18 and older, comparing two vaccine constructs, an optimized B-strain hemagglutinin (HA) candidate (FLUm3HA.b-3NA) and a non-optimized version (FLUm3HA-3NA), against licensed age-appropriate comparators.¹ Adults 18–64 and those 65 and older received different dose levels, with immunogenicity and safety data assessed through day 181 post-vaccination.¹ The optimized candidate generated stronger responses to both influenza A and B strains in younger and older adults respectively, with an acceptable reactogenicity and safety profile.¹ Based on these results, GSK will advance FLUm3HA.b-3NA into a phase III efficacy trial set to begin in September 2026.¹

How does GSK's vaccine differ from other flu shots?

Licensed influenza vaccines primarily target hemagglutinin, the surface protein flu viruses use to bind host cells.¹ GSK's candidate is designed to also target neuraminidase (NA), the second major surface antigen, which the virus uses to spread between cells.¹ Growing evidence suggests that inducing a stronger antibody response to NA, alongside HA, could improve protection and reduce illness severity and transmission.¹ Current inactivated vaccines induce only weak NA-specific immunity, largely because NA is present in low, variable amounts and often loses its natural structure during manufacturing, a gap that has driven interest in NA as a target for broader-coverage flu vaccines.² If GSK's phase 3 trial proceeds as planned, it would be the first late-stage study of an mRNA flu vaccine designed to target both HA and NA.¹

The program builds on GSK's mRNA vaccine collaboration, under which the company gained full rights to develop influenza and COVID-19 mRNA candidates. In July 2026, the FDA granted the candidate fast track designation for the targeted age indication.¹

Why does this matter for the flu vaccine field?

Seasonal influenza causes an estimated one billion cases and up to 650,000 respiratory deaths worldwide each year, with older adults, pregnant women, and people with underlying health conditions at greatest risk.³ GSK's phase 3 plans arrive shortly after Moderna's mFLUSIVA became the first mRNA-based seasonal flu vaccine approved in the US, a milestone that demonstrated the mRNA manufacturing platform built for COVID-19 could support additional commercial respiratory vaccine products. GSK's dual-antigen approach represents a distinct strategy within that same emerging mRNA flu category.

Sanjay Gurunathan, head of vaccines and infectious diseases research and development at GSK, said in a press release, "These promising data support the potential to better protect people from flu with a vaccine designed to target both HA and NA. With one billion flu cases worldwide each year and up to 650,000 deaths there is a need for next generation vaccine approaches. This result is a significant advance in our cutting-edge mRNA program, and we look forward to starting phase III imminently."¹

What happens next?

GSK's phase 3 efficacy trial is scheduled to start in September 2026, evaluating the optimized B-strain HA candidate against licensed comparators. The investigational vaccine is not approved anywhere in the world.¹

References

1. GSK plc. GSK to advance mRNA seasonal flu vaccine candidate to phase III following positive phase II data. Press release. Published September 1, 2026. Accessed September 1, 2026.
2. Strohmeier S, Amanat F, Zhu X, et al. A novel recombinant influenza virus neuraminidase vaccine candidate stabilized by a measles virus phosphoprotein tetramerization domain provides robust protection from virus challenge in the mouse model. mBio. Published online December 2021.
3. World Health Organization. Influenza (seasonal). Accessed September 2026.