CAMBRIDGE, Massachusetts — The message that traveled fastest when Moderna and Merck reported their Phase 3 melanoma vaccine results on August 19 was not contained in a scientific paper. It was not a confidence interval or an absolute risk reduction figure. It was a stock price: Moderna’s shares more than doubled in a single trading session, in a market reaction that priced years of potential ahead of a single data release the companies themselves have not yet submitted for peer review.
For patients with stage three or four melanoma waiting on the results to understand their own treatment options, the distance between that market signal and what the data actually says matters more than it does for any investor.
The vaccine, known as intismeran or mRNA-4157, works by training the immune system to identify and attack cancer cells using messenger RNA derived from mutations in a patient’s own tumor. It is given in combination with Merck’s Keytruda, an immunotherapy that blocks a protein cancer cells use to hide from the immune system. The idea is a two-stage attack: Keytruda removes the disguise; intismeran points the immune system directly at the face underneath.
In a Phase 3 trial of 1,137 patients who had melanoma surgically removed but remained at high risk of recurrence, Moderna and Merck reported a “statistically significant and clinically meaningful improvement” in how long patients remained cancer-free compared with patients who received Keytruda alone. The trial’s primary endpoint, the length of time before disease returned, was met, according to the companies.
What the companies have not said publicly includes the actual percentage by which recurrence was reduced, expressed as an absolute number. What has not been released is a confidence interval for that figure. What has not happened is submission of the full dataset to a peer-reviewed medical journal, where independent statisticians and oncologists could scrutinize the methodology, the patient selection criteria, and whether the improvement holds when examined by people with no financial stake in the answer.
“I’m just excited to see science win on this one,” said Dr. Janice Mehnert, a senior investigator at NYU Langone who was involved in the earlier Phase 2 work, speaking to NBC News. That is a reasonable thing for a researcher to feel in the immediate wake of a positive trial. It is also a statement about emotion, not a statement about data.
The earlier Phase 2 trial followed 157 patients over five years and found that the vaccine halved the risk of melanoma returning. That number was compelling, and it was published. The Phase 3 population is seven times larger, and the results are so far described only in a company press release.
There is also a more structural question that the Phase 3 results have not answered. The experimental group in the trial combined the vaccine with Keytruda; the control group received Keytruda alone. The trial therefore measures the vaccine’s additive effect above Keytruda’s baseline. What it cannot answer is what the vaccine would do without Keytruda, or whether the benefit is concentrated in a subpopulation of patients whose tumors share specific mutation profiles. Personalized mRNA vaccines, by design, are not personalized to every patient the same way. Some patients’ tumors generate more neoantigens than others. The companies have not said whether the benefit in the trial was distributed evenly or driven by a fraction of participants with particular tumor profiles.
None of this means intismeran does not work. The Phase 2 data and the Phase 3 topline announcement together point toward a genuine effect. They do not, yet, tell a clinician or a patient how large that effect is in the real world, who is most likely to benefit, or what the full side-effect profile looks like in a 1,137-person population over a follow-up period longer than the current release covers.
Melanoma kills roughly 8,500 Americans a year. About 112,000 new cases are diagnosed annually. The patients in this trial had already had surgery to remove their tumors but remained at high risk of recurrence, a population for whom better options have been sought for decades. The precedent of researchers working in cancer research long enough to see a Phase 3 trial reach this point is itself a signal that the science behind mRNA-based cancer therapies has moved beyond early proof-of-concept.
The part that is not yet answered is precisely the part that a stock price cannot measure: whether the data, when it is fully published, will support the enthusiasm that arrived four days before the paper did.
Dr. Céline Gounder, a CBS News medical correspondent who has treated melanoma patients, described the results as “a big win for mRNA vaccines against cancer.” As health insurance costs in the United States face their fastest rise in 15 years, a drug that prevents cancer recurrence rather than treating relapse carries its own economic logic beyond the clinical. Fewer courses of treatment for a disease that has returned means lower long-term costs for patients, insurers, and hospital systems.
Moderna has not provided a timeline for the full dataset’s publication or an FDA submission date. The agency reviews applications based on complete datasets, not press releases. What comes next is the paper. That paper is what the Phase 3 trial will ultimately be judged on, and it has not been written yet in a form that oncologists outside of Moderna and Merck have read.
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