A new generation of influenza vaccines may be taking shape. GSK announced on Sept. 1, 2026, that it will advance its investigational mRNA seasonal flu vaccine candidate into a pivotal Phase III trial following positive Phase II results presented at the OPTIONS XIII Conference for the Control of Influenza in Washington, D.C. The trial is set to begin this month.
The development arrives just weeks after Moderna secured U.S. Food and Drug Administration approval for mFlusiva, the first mRNA influenza vaccine to reach market, approved on Aug. 5 for adults 50 and older. GSK’s candidate is pursuing a different and potentially broader strategy, one that has not yet been tested in a Phase III trial anywhere in the world.
A Two-Target Strategy
Most licensed flu vaccines, including Moderna’s newly approved mFlusiva, focus on hemagglutinin (HA), the protein that allows the flu virus to attach to and enter human cells. GSK’s candidate, designated FLUm3HA.b-3NA, is designed to also target neuraminidase (NA), the protein responsible for helping newly replicated virus particles spread from infected cells to healthy ones. Growing evidence suggests that engaging NA alongside HA could improve protection, reduce illness severity, and limit transmission. No approved flu vaccine currently takes this dual approach.
In the Flu-028 Phase II trial, a randomized, observer-blind study enrolling 971 adults aged 18 and older, GSK’s vaccine candidate generated higher immune responses against all influenza strains tested compared to licensed standard-dose and high-dose inactivated flu vaccines, in younger and older adult groups respectively. The trial assessed immunogenicity and safety data through day 181 post-vaccination. GSK has not yet released the underlying numerical data publicly; detailed results were shared in an oral presentation at the OPTIONS XIII conference.
The FDA granted GSK’s candidate Fast Track designation in July 2026 for its targeted age indication, a regulatory tool used to expedite development of therapies addressing unmet medical needs.
An Evolving and Competitive Landscape
Seasonal influenza causes approximately one billion infections annually, including up to five million severe cases and up to 650,000 respiratory deaths worldwide, according to the World Health Organization. Conventional flu vaccines have shown variable effectiveness ranging from roughly 20 to 60 percent over the past 15 years, leaving substantial room for improvement.
Moderna’s mFlusiva demonstrated approximately 26.6 percent greater effectiveness at preventing laboratory-confirmed influenza compared to traditional vaccines in clinical trials, with a stronger immune response in adults 65 and older. The vaccine has been tested in roughly 70,000 clinical trial participants and is now entering its first real-world flu season, with questions about pricing, availability, and uptake still unresolved.
GSK’s entry into Phase III positions it as a second-mover in the mRNA flu space, but with a differentiated scientific rationale. Moderna had previously explored HA and NA dual-targeting in its own pipeline but paused that work amid broader pipeline reprioritization. GSK is now carrying that hypothesis forward into late-stage testing.
Sources and further reading:
GSK to advance mRNA seasonal flu vaccine candidate to phase III following positive phase II data — GSK
GSK catches phase 3 fever after mRNA flu vaccine beats standard shots — Fierce Biotech
How the mRNA flu vaccine compares to traditional shots — Johns Hopkins Bloomberg School of Public Health
This article was researched and sourced by Global Biodefense editors and reported with Claude AI assistance for drafting and editing.