US regulators have licensed MFLUSIVA, the first influenza vaccine of any kind made from messenger RNA. In a trial of 40,703 adults aged 50 and over, confirmed flu illness was 26.6 percent lower than with a licensed standard-dose shot.
Seasonal flu still causes serious illness and death in adults over 50, even among those who get their yearly shot. A trial of 40,703 of them, run across 11 countries through the 2024 to 2025 season, tested whether a different kind of vaccine could narrow that gap. It did: 2.0 percent of those given it caught confirmed flu, against 2.8 percent of those given a standard-dose shot. On 5 August 2026 the US Food and Drug Administration licensed that vaccine, MFLUSIVA, from ModernaTX. It is the first influenza vaccine of any kind built from messenger RNA, the short-lived working copy of a gene that tells a cell which protein to make, and nothing in it is grown: no hens' eggs, no cultured cells, only instructions synthesised outside any living cell for the body to read.
How we know
The trial was called Fluent, and the New England Journal of Medicine published it in May 2026, three months before the licence. It was randomised and double-blind: 20,350 adults aged 50 and over were given the mRNA vaccine, 20,353 a licensed standard-dose one, and neither they nor the staff treating them knew which. What it counted was flu confirmed in a laboratory, meaning fever, chills or aches alongside a respiratory symptom and a swab positive for influenza, from 14 days after the injection to the end of the season. Among the 40,303 participants who followed the protocol throughout, 411 in the mRNA group fell ill against 557 in the comparison group. That is a relative vaccine efficacy of 26.6 percent, which means about a quarter fewer illnesses than the vaccine it was measured against rather than a quarter of all flu prevented, with the range consistent with the data running from 16.7 to 35.4 percent. It cleared not only the bar for being no worse than the standard shot but the two harder bars for being better, both fixed before anyone saw the data. One part of the licence rests on less: for adults 65 and over, the FDA granted the indication under accelerated approval, a licence based on a measurement thought to predict a benefit rather than on the benefit itself, and it has required a confirmatory trial.
Why it matters
Influenza is the disease that has most resisted a lasting vaccine, because it changes shape every year and the shot has to be rebuilt every year to match it. Building it from mRNA changes what rebuilding means: each season's strains become a sequence to rewrite rather than a biological process to run again, and MFLUSIVA's licence already carries an agreed procedure for swapping them. The version approved holds three, chosen by the FDA for the 2026 to 2027 season. What it does not yet show is a benefit for the oldest patients: the relative vaccine efficacy was measured against a standard-dose shot, which was not the vaccine US authorities preferred for people over 65, and the accelerated approval covering that age group stands on antibody levels until the required trial reports in 2030. The platform has proved it can beat the standard shot. Whether it protects the people influenza hurts most is the next thing to find out.
The 26.6 percent is a comparison between two vaccines, not a measure of protection against flu, and what the trial counted was confirmed illness rather than hospital admission or death. For adults 65 and over, nearly half of the participants, the licence rests on antibody responses measured in a separate study rather than on evidence that the vaccine prevented illness in that age group, and the FDA has required a trial against a high-dose vaccine, starting in August 2026 and not reporting in full until 2030. Side effects were more common than with the standard shot, with injection-site pain in 65.8 percent of recipients against 29.8 percent and fatigue in 45.1 percent against 20.3 percent, though most were mild or moderate and lasted about two days. The trial was funded by Blackstone Life Sciences and Moderna, and people with weakened immune systems were excluded from it.
Does this mean the vaccine prevents a quarter of flu cases?
No. The 26.6 percent figure compares the new vaccine with an existing licensed standard-dose one, so it means about a quarter fewer confirmed illnesses than that vaccine produced, among people already partly protected by it. In the trial, 2.0 percent of those given the new vaccine caught confirmed flu, against 2.8 percent of those given the standard shot.
Why is the licence for people 65 and over different?
The FDA granted that part of the indication on antibody responses measured in a separate study, rather than on evidence that the vaccine prevented illness in that age group. It has required Moderna to run a confirmatory trial against a high-dose vaccine, beginning in August 2026, with the final report due in 2030. If that trial does not verify the benefit, the agency can withdraw the approval or narrow what the vaccine is licensed for.
Were the side effects worse than with a standard flu shot?
They were more common. Injection-site pain was reported by 65.8 percent of those given the new vaccine against 29.8 percent given the standard shot, fatigue by 45.1 percent against 20.3 percent, and muscle aches by 35.4 percent against 11.6 percent. Most reactions were mild or moderate, started about two days after the injection and lasted about two days.
Is it made in hens' eggs?
No. Neither eggs nor cultured cells are used to make it, and it contains no egg protein. Its active ingredient is a set of genetic instructions, synthesised outside any living cell, that the body reads to build three influenza surface proteins for the immune system to learn.
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