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Traditional flu vaccines are typically grown using egg based or cell based methods, a process that takes months. mRNA vaccines work differently. Instead of growing a weakened or inactivated virus, scientists create a small snippet of genetic instructions that teaches the body’s own cells to produce a harmless piece of the flu virus. The immune system then learns to recognize and fight that piece, building protection without ever encountering the actual virus. This approach became widely known during the COVID pandemic. Applying it to influenza has been an active research goal for years, since flu strains shift constantly and manufacturers need to keep up.
This article was created with the assistance of AI and reviewed by our editorial team for accuracy and clarity.
How mRNA Vaccines Actually Work
Traditional flu vaccines are typically grown using egg based or cell based methods, a process that takes months. mRNA vaccines work differently. Instead of growing a weakened or inactivated virus, scientists create a small snippet of genetic instructions that teaches the body’s own cells to produce a harmless piece of the flu virus. The immune system then learns to recognize and fight that piece, building protection without ever encountering the actual virus. This approach became widely known during the COVID pandemic. Applying it to influenza has been an active research goal for years, since flu strains shift constantly and manufacturers need to keep up.
The Trial Involved More Than 40000 People
The vaccine’s approval rests on a large study called FLUENT, which enrolled over 40,000 adults aged 50 and older across 301 sites in 11 countries. Participants were randomly assigned to receive either the new mRNA vaccine or one of several standard dose flu shots already on the market. Researchers then tracked who developed confirmed influenza over the course of a full flu season. Trials of this size are unusual for flu vaccines and were designed specifically to give regulators enough data to compare real world effectiveness between the new technology and the shots people have relied on for years.
It Outperformed the Standard Shot by 27 Percent
The headline result: about 2 percent of people who got the mRNA vaccine developed flu illness, compared with 2.8 percent in the standard shot group. That translates to a relative vaccine efficacy of roughly 26.6 percent, meaning the mRNA shot prevented significantly more cases than the traditional vaccine it was tested against. Flu shot effectiveness varies considerably from year to year, sometimes dropping as low as 20 to 30 percent depending on how well the vaccine matches circulating strains. A vaccine that consistently outperforms the standard option, even by this margin, could meaningfully reduce flu related illness among older adults.
Right Now It Is Only for Older Adults
The FDA’s approval is not universal. It covers two specific groups: adults aged 50 to 64 and adults 65 and older. The younger group received standard, full approval based on the trial data. The older group received what is known as accelerated approval, a conditional pathway that requires Moderna to run an additional confirmatory trial after the vaccine reaches the market. This staged approach reflects both the strength of the existing data in older adults and regulators’ desire for more evidence specifically in the oldest and most vulnerable patients before broadening the vaccine’s use further.
Side Effects Were More Common but Mild
Regulators found no new or serious safety concerns during their review. However, people who received the mRNA vaccine reported more solicited side effects than those who got the standard shot. The most common were injection site pain, fatigue, and headache, all consistent with reactions seen in other mRNA vaccines like the COVID shots. These effects were described as mostly mild to moderate and short lived. An FDA advisory panel reviewed the safety data and voted unanimously in favor of the vaccine, with one panelist calling the trial results clear and robust during the public meeting.
The Approval Almost Did Not Happen This Year
This vaccine’s path to approval was unusually turbulent. Early this year, the FDA initially declined to even review Moderna’s application, a rare move that drew criticism from clinicians and public health experts. The agency reversed course about a week later and agreed to proceed with review ahead of the 2026 to 2027 flu season. The reversal came against a backdrop of broader skepticism toward mRNA vaccines from federal health leadership since the COVID pandemic. Despite that friction, the review ultimately moved forward on the strength of the clinical trial data.
Why Speed Matters for Future Flu Seasons
One of the biggest advantages of mRNA technology has nothing to do with this year’s approval. It is about the future. Traditional flu vaccines take around six months to manufacture once scientists select which strains to target. mRNA vaccines could potentially cut that timeline down to two or three months. That matters because flu strains can shift unpredictably, and a shorter production window means vaccines could be updated later in the year with a better match to whatever strain is actually circulating, potentially improving effectiveness in seasons when predictions miss the mark.
When and How You Might Be Able to Get It
Moderna says mFlusiva will be available in the United States in time for the 2026 to 2027 flu season, meaning eligible adults could see it at pharmacies and clinics this fall. There is a wrinkle, though. Vaccines are typically formally recommended by the CDC’s independent vaccine advisory committee before widespread use, but a federal judge has blocked that committee from meeting. That leaves some uncertainty about how quickly formal recommendations and insurance coverage guidance will catch up with the FDA’s approval, even though the vaccine itself is now authorized.
What This Approval Really Means Going Forward
The approval of mFlusiva marks the first time mRNA technology has been authorized for a flu vaccine anywhere in the world, not just in the United States. For adults 50 and older, it offers an option shown to outperform standard shots in a large, rigorously designed trial. The bigger story may be what comes next. If the technology proves itself further, it could eventually expand to younger age groups and pave the way for flu vaccines that are updated faster and matched more precisely to each year’s dominant strains, changing how seasonal flu protection works for everyone.