CATEGORY: IMMUNE HEALTH
An mRNA flu vaccine produced 26.6% fewer confirmed flu illnesses than a standard flu shot in a randomized trial of 40,805 adults aged 50 and older. That came to 411 cases, or 2.0%, among mRNA recipients against 557 cases, or 2.8%, with the standard vaccine, across the 2024-2025 flu season.1 The results were published in the New England Journal of Medicine. In August 2026 the FDA approved the vaccine, called mFlusiva, for adults 50 and older, the first mRNA influenza vaccine to reach the market. The shot this trial tested is now something a pharmacist can draw up.
From Chicken Eggs to Genetic Instructions
Most flu vaccines are still made the way they have been for decades, by growing the virus in chicken eggs. That takes months, and it locks in the choice of strains long before flu season arrives. mRNA vaccines deliver something else entirely: a short-lived genetic instruction sheet that teaches your own cells to build one harmless flu protein, which the immune system then learns to recognize. No eggs, no live virus. Production runs faster, which in principle lets vaccine makers wait longer before committing to a season’s strains. The trial’s authors cited the platform’s promise of better protection in older adults as their reason for running the comparison.
Symptom Diaries Twice a Week, Then a Nose Swab
Moderna and Blackstone Life Sciences funded the trial, which ran across 11 countries and gave each of its 40,805 participants either the mRNA vaccine or a licensed standard-dose flu shot. Which one you got was decided at random. Surveillance was active rather than passive. Participants filled in symptom diaries twice a week, and anyone with flu-like symptoms had a nose swab within 72 hours, tested by RT-PCR, a laboratory method that detects the virus’s genetic material. A case counted only if the virus was confirmed, starting two weeks after vaccination and running to the season’s end.
Eight Fewer Cases in Every Thousand
Confirmed flu illness struck 2.0% of the mRNA group against 2.8% of the standard-shot group. In round numbers, 20 people in every 1,000 rather than 28. That works out to 26.6% fewer cases relative to a vaccine that already works, with a confidence range of 16.7% to 35.4%. The trial had two prespecified bars to clear, in order: first that the new vaccine matched the standard shot, then that it beat it. It cleared both. Among the 19,260 participants aged 65 and older, the advantage held at 27.4%.
The Sore Arms, and the Comparison Nobody Has Run
The stronger immune response cost something in comfort. Injection-site pain was reported by 65.8% of mRNA recipients against 29.8% with the standard shot, and fatigue, headache and muscle aches were all more common. Serious adverse events were similar, 2.2% against 1.9%, and no cases of myocarditis or pericarditis, the heart-inflammation conditions watched for with mRNA vaccines, were identified in the 42-day monitoring window. One question the trial left open, and the authors say so plainly: the comparator was a standard-dose shot, while several countries already recommend enhanced flu vaccines for adults 65 and older, either high-dose or immune-boosted versions. A 2014 randomized trial of 31,989 older adults found that the high-dose vaccine itself beat the standard dose by about 24%, a similar margin to the mRNA result.2 The head-to-head test between mRNA and those enhanced shots has not been run. The FDA has also required a follow-up study of the new vaccine’s safety and effectiveness after launch.
Key Takeaways
- Over the 2024-2025 season, an mRNA flu vaccine produced 26.6% fewer lab-confirmed flu illnesses than a standard-dose shot among 40,805 adults: 20 cases per 1,000 against 28. In the 19,260 participants aged 65 and older the margin was 27.4%.
- Temporary side effects such as arm pain, fatigue and headache were notably more common with the mRNA vaccine, while serious adverse events were similar between the groups.
- The comparison was against standard-dose shots only, not against the high-dose or immune-boosted vaccines many countries already recommend for adults 65 and older.