GSK is moving its experimental mRNA-based seasonal flu vaccine into a late-stage trial after mid-stage data showed it generated stronger immune responses than conventional shots, positioning the British drugmaker to challenge Moderna’s recently approved mRNA flu product.

The decision follows results from the Flu-028 study, which were presented Monday at the OPTIONS XIII Conference for the Control of Influenza in Washington, D.C. The Phase IIa trial randomized 971 adults into groups receiving either one of two GSK mRNA candidates or a licensed comparator, including standard and high-dose flu vaccines. The company said its shots produced higher immune responses than the marketed vaccines in both younger adults aged 18 to 64 and older adults 65 and above, with an acceptable safety and reactogenicity profile.

GSK did not disclose specific figures behind the efficacy conclusions in its announcement. The two mRNA candidates tested differed in how they optimized the B-strain hemagglutinin (HA) protein. Based on the data, the company selected FLUm3HA.b-3NA, the candidate with the optimized B-strain HA, for further development.

A Phase III study is expected to begin this month, September 2026. If it launches on schedule, GSK will become the first company to evaluate an mRNA flu vaccine targeting both HA and neuraminidase (NA) in late-stage testing.

“This approach could help better protect people from flu,” said Sanjay Gurunathan, GSK’s head of vaccines and infectious disease R&D, citing the vaccine’s capacity to target two primary flu surface proteins that play key roles in the virus’s ability to bind and enter host cells.

The rationale behind the dual-target strategy stems from the limitations of existing flu shots, which primarily target HA alone. Licensed vaccines have shown effectiveness ranging from 20% to 60% over the past 15 years. Evidence suggests that engaging NA, a second surface protein, can broaden and strengthen protection against influenza.

Market context and competitive landscape

The global flu vaccine market represents a multibillion-dollar opportunity, with an estimated one billion cases and 650,000 deaths worldwide linked to influenza each year. GSK’s own flu vaccine business contracted by roughly 25% last year amid what the company described as competitive pressure in the U.S.

Moderna secured FDA approval in August 2026 for mFlusvia, the first mRNA-based seasonal flu vaccine to reach the U.S. market. That product encodes for three HA proteins. Moderna had previously tested vaccine candidates encoding both HA and NA in human trials, believing the combination could broaden protection, but paused that work during a pipeline restructuring.

GlobalData, the parent company of Clinical Trials Arena, forecasts mFlusvia will generate $831 million in sales by 2032. GSK’s entry into the market would create direct competition in the mRNA flu space, with the British company’s NA-targeting approach serving as a key differentiator.

Moderna’s path to approval was not without obstacles. The FDA initially refused to review the vaccine entirely before ultimately granting clearance, capping a tumultuous regulatory journey.

Broader mRNA landscape

The mRNA platform has faced significant political headwinds in the United States. Health Secretary Robert F. Kennedy Jr. has long questioned the technology’s safety profile. In August 2025, he publicly cut government funding to 22 mRNA vaccine development initiatives worth $500 million. Private equity financing for the modality subsequently dried up, declining 82% in 2025 compared to 2023.

Despite these challenges, some analysts see signs of renewed interest. Moderna’s landmark Phase III success for its mRNA-based cancer vaccine, intismeran autogene, in melanoma has drawn attention for its potential to offer additive efficacy over standard-of-care Keytruda (pembrolizumab) alone in the adjuvant setting.

However, Jefferies senior equity research analyst Andrew Tsai cautioned that investors were unlikely to immediately return to mRNA-based therapies. Long-term interest, he said, would likely hinge on the drug class’s success across a broader range of solid tumors.

For GSK, the Phase III readout will be critical in determining whether the company can re-energize its flu franchise and capture a meaningful share of the evolving mRNA vaccine market.