GSK to move mRNA seasonal flu vaccine candidate into phase III testing following encouraging phase II results

GSK plc presented positive phase II results for its mRNA seasonal influenza vaccine candidate at the OPTIONS XIII Conference for the Control of Influenza. The vaccine candidate produced stronger immune responses against every influenza strain tested compared with licensed standard dose and high dose inactivated flu vaccines, in younger and older adults respectively, and was generally well tolerated. Based on these findings, GSK plans to begin a phase III efficacy trial in September 2026.

This will mark the first phase III trial of an mRNA flu vaccine built to target both haemagglutinin and neuraminidase, the two primary surface proteins that allow the flu virus to bind to cells and spread. While currently licensed flu vaccines mainly target haemagglutinin, GSK’s candidate is specifically designed to also target neuraminidase. A growing body of evidence suggests this dual approach could help strengthen protection while reducing illness severity and transmission.

Sanjay Gurunathan, GSK’s head of vaccines and infectious diseases research and development, said these encouraging results support the potential for better flu protection through a vaccine designed to target both haemagglutinin and neuraminidase. He noted that with an estimated one billion flu cases occurring worldwide each year and up to 650,000 associated deaths, there is a clear need for next-generation vaccine approaches. He described the result as a meaningful step forward for the company’s mRNA vaccine program and said GSK is looking forward to launching phase III testing soon.

In July 2026, the U.S. Food and Drug Administration granted the vaccine candidate Fast Track designation for its targeted age group, reflecting the pressing need to improve protection against flu.

About the Flu-028 phase II trial

Flu-028 is a randomized, observer-blind phase II trial evaluating the immunogenicity and safety of mRNA-based multivalent seasonal flu vaccine candidates in 971 adults aged 18 and older. Participants aged 18 to 64 and those aged 65 and older received varying dose levels of either FLUm3HA.b-3NA, which encodes an optimized B-strain haemagglutinin, FLUm3HA-3NA, which encodes a non-optimized B-strain haemagglutinin, or licensed comparators appropriate for their age group. Immunogenicity and reactogenicity and safety data were assessed through day 181 after vaccination.

Earlier research had identified gaps in existing flu vaccination approaches, including the underappreciated role of neuraminidase and the need to strengthen immune responses to B-strain haemagglutinin. In the Flu-028 phase II trial, GSK’s FLUm3HA.b-3NA vaccine candidate generated strong immune responses to both haemagglutinin and neuraminidase from influenza A and B strains in both younger and older adults, with acceptable reactogenicity and safety profiles. Based on these results, GSK will continue investigating the optimized B-strain haemagglutinin candidate, FLUm3HA.b-3NA, in a phase III efficacy trial.

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Related Topics and Keywords

flu vaccine, GSK, mRNA seasonal flu vaccine, phase II results, Phase III trial