GSK advances mRNA flu vaccine to phase III after 971-patient trial

GSK plc will move its mRNA seasonal influenza vaccine candidate into a phase III efficacy trial starting September 2026, after phase II data showed higher immune responses than licensed standard- and high-dose shots in younger and older adults respectively.

"These promising data support the potential to better protect people from flu with a vaccine designed to target both HA and NA," Sanjay Gurunathan, GSK Head of Vaccines and Infectious Diseases R&D, said. "This result is a significant advance in our cutting-edge mRNA programme, and we look forward to starting phase III imminently."

The Flu-028 trial enrolled 971 adults aged 18 and older in a randomized, observer-blind study assessing immunogenicity and safety of multivalent mRNA flu vaccine candidates. GSK's FLUm3HA.b-3NA candidate, encoding an optimized B-strain HA, generated robust immune responses to haemagglutinin and neuraminidase from influenza A and B strains, with acceptable reactogenicity and safety profiles through day 181. The candidate is designed to target both HA and NA, the primary surface antigens that cause the flu virus to bind and spread, whereas licensed flu vaccines primarily target HA.

The US Food and Drug Administration granted the vaccine candidate Fast Track designation in July 2026 for the targeted age indication. Seasonal flu causes about one billion cases globally each year, including up to five million cases of severe illness and up to 650,000 respiratory deaths, according to the World Health Organization. The phase III trial will be the first of an mRNA flu vaccine designed to target both HA and NA, a design GSK says could improve protection, illness severity and transmission.

The advancement strengthens GSK's vaccine pipeline in a seasonal flu market worth billions annually and intensifies competition with Moderna and Pfizer, which have also pursued mRNA flu programs. Investors will watch the phase III efficacy readout, expected after the September 2026 trial start, for confirmation that the dual-antigen approach translates into real-world protection.

This article is for informational purposes only and does not constitute investment advice.