GSK moves mRNA flu vaccine toward phase III after stronger immune responses in Phase II

GSK is preparing to move its investigational mRNA seasonal influenza vaccine into Phase III development after reporting positive Phase II findings in both younger and older adults.

The candidate generated stronger immune responses against all influenza strains assessed when compared with licensed inactivated vaccines. In younger adults, responses were higher than those seen with a standard dose flu vaccine, while in older adults the candidate outperformed a high dose flu vaccine. The vaccine was also generally well tolerated.

Following the results, GSK plans to begin a Phase III efficacy study in September 2026.

The investigational vaccine has been designed to produce immune responses against both haemagglutinin, known as HA, and neuraminidase, known as NA. These are two major proteins on the surface of influenza viruses that play roles in viral attachment and spread.

Most currently licensed influenza vaccines primarily target HA. GSK’s candidate has been developed to target NA as well, an approach the company says could potentially strengthen protection while also influencing disease severity and transmission.

Sanjay Gurunathan, Head of Vaccines and Infectious Diseases Research and Development at GSK, said the Phase II results support the potential of a vaccine targeting both HA and NA to improve protection against influenza. He highlighted the global burden of the disease, with around one billion cases occurring annually and as many as 650,000 deaths, and said the findings represent an important step forward for GSK’s mRNA programme ahead of the planned Phase III study.

The US Food and Drug Administration granted Fast Track designation to the vaccine candidate in July 2026 for the targeted age indication. The designation is intended to support the development and review of treatments and vaccines addressing serious conditions where there is an unmet medical need.

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