Moderna Inc. (NASDAQ: MRNA) shares fell 17.6% on Thursday, and extended losses in Friday premarket trading after detailed results emerged for its experimental cancer vaccine developed with Merck (NYSE: MRK). The Phase 3 trial showed that the treatment reduced the risk of melanoma recurrence or death by 42% when combined with Keytruda, but investors sold shares after a rally of more than 500% this year.
Moderna closed Thursday at $202.63, down from $245.81 a day earlier, after falling as low as $180.80 during the session. Shares then dropped a further 4% in Friday premarket trading at one point.
The selloff came despite encouraging clinical results. The treatment, called intismeran autogene, is a personalized messenger RNA (mRNA) therapy designed to help the immune system recognize and attack cancer cells. The findings strengthen Moderna’s effort to expand beyond its established vaccine business, but they do not guarantee regulatory approval or future revenue.
What Moderna’s New Cancer Vaccine Does and Why the Results Matter
Moderna and Merck tested intismeran autogene alongside Keytruda, Merck’s established cancer immunotherapy, in the Phase 3 INTerpath-001 trial. The study enrolled 1,137 patients with high-risk stage IIB to IV melanoma whose tumors had been surgically removed.
Unlike a conventional preventive vaccine, intismeran is a therapeutic cancer vaccine intended to reduce the risk of the disease returning after surgery. Researchers analyze mutations in a patient’s tumor to develop a personalized mRNA treatment that trains the immune system to recognize tumor-specific targets. The treatment is administered alongside Keytruda, which helps the immune system attack cancer.
In the trial, the combination reduced the relative risk of melanoma recurrence or death by 42% compared with Keytruda alone. After two years, 82.2% of patients receiving both treatments remained alive without a recurrence, compared with 70% of those treated with Keytruda alone. The median follow-up was 24.6 months.
The combination also reduced the risk of distant metastasis or death by 38%. Distant metastasis occurs when cancer spreads to organs or tissues away from its original site. At the two-year mark, distant metastasis-free survival stood at 88.5% for the combination group, compared with 80.2% among patients receiving Keytruda alone.
The detailed Phase 3 results reported by Reuters confirm that the trial met its primary endpoint of recurrence-free survival and a key secondary endpoint measuring the risk of cancer spreading.
The results are clinically important, but the distinction between relative risk reduction and absolute outcomes matters. A 42% reduction in risk does not mean 42% more patients were cured. Instead, it measures how much lower the risk of recurrence or death was in the treatment group relative to the control group.
The trial also has limitations. Overall survival data are not yet mature, meaning researchers do not have enough evidence to determine whether the combination improves patients’ overall lifespan. The study remains ongoing, and the companies reported a favorable trend in survival outcomes rather than a definitive benefit.
The detailed results were originally scheduled for presentation at the European Society for Medical Oncology Congress in Madrid on Saturday, October 24. However, the data appeared prematurely on an oncology news website on Thursday. The apparent embargo breach prompted the companies to release the findings early, according to STAT’s report on the clinical data.
Why Moderna Stock Fell and What the Vaccine Means for Its Future
The share-price decline appears to reflect investor expectations rather than a failure of the trial. Moderna stock had climbed more than 500% in 2026 as investors anticipated evidence that its cancer vaccine could become a major commercial product. The positive results broadly supported the treatment’s prospects, but some investors had hoped for an even stronger outcome.
Analysts also raised questions about whether the therapy will work equally well across other cancer types. Moderna and Merck are testing personalized mRNA treatments beyond melanoma, including in other solid tumors, but success in one cancer does not establish effectiveness in another. The companies have also noted that the treatment reduced the risk of distant metastasis by 38%, below the 59% reduction reported in an earlier five-year Phase 2 analysis.
The selloff illustrates how clinical-trial milestones can affect biotech valuations. A company’s share price may rise well before detailed trial data become available, leaving little room for results that meet expectations without substantially exceeding them. Moderna was already among the healthcare stocks that outperformed the wider market in 2026, making the size of the rally an important part of the market reaction.
For Moderna, the potential commercial opportunity is significant. The company has been trying to establish new sources of growth beyond its COVID-19 vaccine franchise. A successful personalized cancer treatment could give it a position in oncology, an area in which Merck already has a major presence through Keytruda.
The path to revenue, however, remains uncertain. Intismeran is still investigational and has not received US Food and Drug Administration approval. The companies said they are engaging with regulators on potential submissions, but regulatory timing and the eventual launch date have not been confirmed. Its commercial prospects will depend on approval, reimbursement, manufacturing costs and whether clinicians adopt the treatment for eligible patients.
The therapy’s individualized nature also creates practical challenges. Each vaccine is developed to match a patient’s tumor mutations, requiring testing and production tailored to that individual. The process must work reliably at scale if the product is to reach a large patient population.
Safety remains another factor for regulators and investors. The companies reported no new safety signals in the Phase 3 trial, and the treatment’s safety profile was generally consistent with earlier studies. However, treatment-related adverse events were reported in 95.8% of patients receiving the combination, compared with 90% in the Keytruda-only group. Severe treatment-related adverse events occurred in 14.2% of the combination group and 13.4% of the control group.
The vaccine’s broader impact will depend on whether further research confirms its benefits and whether the companies can turn those benefits into an approved, commercially viable treatment. The results give Moderna a credible oncology opportunity, but they are not yet evidence of a new revenue stream.
For investors, the central question is whether the cancer vaccine can support Moderna’s valuation after the dramatic stock rally. The clinical data offer meaningful evidence of efficacy in high-risk melanoma, but commercialization and long-term outcomes remain unresolved. Thursday’s selloff shows that even encouraging medical results can trigger a sharp repricing when investor expectations are exceptionally high.