GSK's mRNA Flu Vaccine: Promising Antibodies, Unproven Protection

On 1 September, GSK said it would push its experimental messenger-RNA seasonal flu vaccine into a Phase III efficacy trial starting this month, after a mid-stage study of 971 adults produced higher antibody responses than licensed standard- and high-dose flu shots. The candidate, called FLUm3HA.b-3NA, is the first mRNA flu vaccine to reach late-stage testing while aiming at two of the virus's surface proteins rather than one. That combination is what makes the news worth a second look — and also why the part investors actually care about is still unresolved.

The late-stage test is efficacy, not antibodies

The first thing to note about that Phase II result is what it did and did not measure. Higher immune responses means higher antibody titers against the strains tested — a proxy for protection, not proof of fewer infections. The test that decides whether this vaccine is worth anything commercially is efficacy: does it prevent laboratory-confirmed flu better than the shots people already receive? That metric has defined, and constrained, the entire mRNA-flu category.

Every mRNA flu program has hit the same wall at the efficacy stage, and the bar itself is low. Conventional flu vaccines have delivered roughly 20% to 60% effectiveness over the past 15 years. Moderna's mRNA shot, mFlusiva, which the FDA approved on 5 August for adults 50 and older as the first product of its kind, showed relative efficacy of about 26.6% in a trial covering roughly 70,000 people — meaning flu illness fell by about a quarter relative to traditional shots. It beat the standard comparator, but not by a wide margin. The commercial reading is telling: one forecaster expects mFlusiva to reach only about $831 million in revenue by 2032, a modest product inside a flu market worth roughly $9 billion to $10 billion globally today.

One mechanism could change the outcome

GSK is betting its dual-antigen design improves on that arithmetic. Standard and mRNA flu vaccines target haemagglutinin (HA), the surface protein the virus uses to latch onto cells. GSK's candidate also targets neuraminidase (NA), the protein that helps newly formed virus spread and exit cells. The hypothesis, supported by a growing body of evidence, is that adding NA yields better protection, less severe illness, and reduced onward transmission. No approved flu vaccine uses both, and Moderna had paused its own similar dual-targeting work — leaving GSK the only company testing the approach late-stage. That is a credible reason to think GSK's efficacy numbers could come in better than the field's, and the strongest argument for keeping the Phase III readout on a watchlist.

A small, distant bet on a dividend business

But here is what the headliner leaves out. Even if Phase III succeeds, this is a small and distant bet relative to the company behind it. GSK acquired the flu candidates outright from CureVac in July 2024 for €400 million upfront plus up to €1.05 billion in milestones and royalties — pocket change against a business that booked £32.7 billion of sales in 2025. Vaccines brought in £9.2 billion of that, led by the shingles shot Shingrix at £3.6 billion, with flu one line within the category. A Phase III efficacy trial that begins this month and is designed to show a difference across a flu season or two will realistically read out in 2028 at the earliest, pushing any commercial launch toward the end of the decade.

The engine that supports GSK's value case does not depend on this pipeline shot. GSK trades around 11 times forward earnings with a dividend yield near 3.7% and a payout close to 45% of earnings. It raised its 2025 dividend to 66p a share and guided to 70p for 2026, and it generated £8.9 billion of cash from operations in 2025 against that modest payout — comfortable cover. Management's longer-term target is more than £40 billion of sales by 2031, built on franchises it already owns rather than an unapproved flu vaccine.

The useful way to hold this news, then, is as free optionality on a stock that stands on its own. The Phase II data clears the candidate for the test that actually determines its value, and GSK has a differentiated mechanism the rest of the field lacks. But that test — relative efficacy against licensed shots in a large trial — is years away and has humbled every competitor that ran it first. For the investor deciding what this changes about GSK, the honest answer is very little: the dividend and franchise case never needed it. Watch the efficacy readout when it lands; do not buy the stock on the premise that it will be good.