GSK's mRNA Influenza Vaccine Moves to Phase 3 Trial

GSK presented positive phase 2 results for its investigational mRNA seasonal influenza vaccine candidate at the OPTIONS XIII Conference for the Control of Influenza. The company said the shot generated stronger immune responses against every influenza strain tested than currently licensed flu vaccines, while remaining generally well tolerated, and that it now plans to move the program into a phase 3 trial.

Phase 2 Data

The results come from Flu-028, a randomized, observer-blind phase 2 trial that assessed the immunogenicity and safety of GSK's multivalent mRNA flu vaccine candidates in 971 adults age 18 and older. Adults ages 18–64 and those 65 and older received different dose levels of one of two investigational candidates, FLUm3HA.b-3NA or FLUm3HA-3NA, or a licensed age-appropriate comparator vaccine. Researchers tracked immunogenicity and safety data through day 181 after vaccination.

Compared with licensed standard-dose vaccines in the younger adult group and high-dose vaccines in the older adult group, GSK's candidates produced higher immune responses across all influenza strains studied. The FLUm3HA.b-3NA candidate, which encodes an optimized B-strain HA component, showed robust immune responses to HA and NA from both influenza A and B strains in both age groups, with an acceptable reactogenicity and safety profile. Based on those results, GSK selected FLUm3HA.b-3NA to carry forward into phase 3 testing.

About the Vaccine

GSK's mRNA seasonal flu vaccine program is built to target both HA and NA, the two primary surface antigens that allow influenza viruses to bind to and infect cells and spread between people. Most licensed flu vaccines are designed primarily around HA, and growing evidence suggests that also targeting NA could improve protection, reduce illness severity, and limit transmission.

The upcoming phase 3 trial, expected to begin this month, will mark the first phase 3 study of an mRNA flu vaccine designed to target both HA and NA. The program sits within GSK's broader respiratory vaccines research effort, and the company emphasized that the candidate is not yet approved anywhere in the world. In July 2026, the FDA granted the vaccine Fast Track designation for its targeted age indication, a status meant to speed development and review of therapies addressing unmet medical needs.

Sanjay Gurunathan, GSK's head of vaccines and infectious diseases research and development, said the data reflect meaningful progress for the company's broader effort to modernize flu prevention.

"These promising data support the potential to better protect people from flu with a vaccine designed to target both HA and NA," Gurunathan said. "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 3 imminently."

Reference

GSK to advance mRNA seasonal flu vaccine candidate to phase III following positive phase 2 data. GSK press release. Published September 1, 2026. Accessed September 1, 2026. https://www.gsk.com/media/dsebith3/options_phase-ii-data_stock-exchange-announcement.pdf