Moderna Confirms New Flu Shot Causes Six Times More Severe Reactions

Written by Jon Fleetwood on May 21, 2026. Posted in Current News

A newly published New England Journal of Medicine Phase 3 trial has confirmed that Moderna’s experimental mRNA seasonal influenza vaccine, mRNA-1010, caused severe short-term reactions at roughly six times the rate of standard-dose flu shots while delivering less than a 1-percentage-point absolute reduction in symptomatic, PCR-confirmed influenza-like illness

The study found that recipients of the mRNA flu shot experienced dramatically more severe short-term side effects, while the overall difference in influenza illness between the two groups remained small.

The trial, published on Thursday, evaluated more than 40,000 adults aged 50 and older across 301 clinical sites in 11 Northern Hemisphere countries during the 2024–2025 influenza season.

The new study compared Moderna’s trivalent mRNA-1010 shot against licensed standard-dose influenza vaccines, not the enhanced high-dose, adjuvanted, or recombinant flu vaccines already recommended for many older adults.

That distinction is important because the study did not prove Moderna’s shot performed better than the said-to-be strongest influenza vaccines currently available for seniors. Instead, it showed alleged superiority only against older standard-dose flu shots.

Researchers reported that PCR-confirmed, protocol-defined influenza-like illness occurred in 2.0 percent of mRNA-1010 recipients compared with 2.8 percent of standard-dose flu shot recipients—a 0.8-percentage-point absolute difference that the paper framed as 26.6 percent relative vaccine efficacy.

For the average reader, that means the “26.6 percent better” headline sounds much larger than the actual real-world difference observed in the study population.

More than 97 percent of participants in both groups did not develop the study’s primary endpoint.

At the same time, Grade 3 solicited adverse reactions—severe reactions that interfere with normal activity—occurred in 6.4 percent of mRNA-1010 recipients compared with 1.0% of standard-dose flu shot recipients.

The mRNA group also experienced sharply higher rates of fatigue, headache, muscle pain, chills, and injection-site pain. That means approximately one in 16 recipients of the mRNA flu shot experienced a severe short-term reaction serious enough to disrupt daily activity, compared with roughly one in 100 recipients of standard flu shots.

The study did not prove reductions in death, ICU admission, or major influenza complications. Hospitalizations were reported in only four mRNA recipients and eight comparator recipients, and the authors acknowledged the trial was not powered for those outcomes.

Meaning the study was not designed to determine whether the mRNA flu shot meaningfully reduced severe influenza outcomes such as hospitalization or death.

The study was funded by Blackstone Life Sciences and Moderna, and Moderna was responsible for the trial design, site selection, monitoring, and data analysis. The manuscript was also prepared by Moderna employees with help from medical writers funded by Moderna.

In other words, the company developing the product also designed the study, analyzed the data, and helped write the final paper presenting the results.

That raises conflict-of-interest concerns, but also means the company itself has confirmed that its experimental mRNA flu shot produced dramatically higher severe reaction rates while delivering only a small absolute reduction in symptom-defined, PCR-confirmed influenza illness.

Moderna was created through funding from the Defense Advanced Research Projects Agency (DARPA), a secretive U.S. military research arm with little public oversight and a history (here) of dual‑use experimentation.

Moderna’s own Phase 3 trial showed its experimental mRNA flu shot caused dramatically higher severe reaction rates while delivering only a small absolute reduction in symptom-defined, PCR-confirmed influenza illness—without proving meaningful reductions in hospitalization, death, or the most dangerous influenza outcomes.

The study also avoided comparison against both a true saline placebo and the strongest enhanced flu vaccines already recommended for seniors, meaning the trial never established whether the product meaningfully outperformed either no flu shot at all or the purportedly “best” existing influenza vaccines.

See more here substack.com

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Header image: Ruby Wallau for STAT

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