Moderna Inc. (MRNA) stock plunged more than 25% Thursday after the alleged leak of an ESMO 2026 conference abstract raised concerns about the effectiveness of its experimental personalized melanoma vaccine.

The reported Phase 3 results suggested a smaller treatment benefit than an earlier study. The data also raised questions about whether the vaccine offers additional advantages over existing immunotherapy approaches.

Moderna and Merck & Co. Inc. (MRK) are developing intismeran autogene, an mRNA-based cancer therapy tailored to mutations in individual patients' tumors.

The companies are scheduled to present detailed results from their Phase 3 INTerpath-001 trial at the ESMO 2026 Congress.

The study evaluates intismeran combined with Merck's Keytruda (pembrolizumab) against Keytruda alone in patients with stage IIB-IV melanoma whose tumors have been surgically removed.

Alleged Phase 3 Leak Raises Questions

The allegedly leaked data indicated that intismeran plus Keytruda reduced the risk of cancer recurrence or death by 42%, with a hazard ratio of 0.58.

Although the reported result suggests a meaningful treatment benefit, it was below the 49% risk reduction observed in an earlier Phase 2b study.

The findings also sparked debate over whether the personalized vaccine offers sufficient additional value compared with administering existing immunotherapy before surgery.

In an X post, biotech commentator Adam May highlighted that the reported hazard ratio matched the 0.58 figure from the separate SWOG S1801 melanoma study.

That study found a 42% reduction in the risk of an event when patients received Keytruda before and after surgery rather than only after surgery.

However, the trials evaluated different treatment strategies and endpoints. Therefore, their results do not establish equivalent effectiveness.

The alleged Phase 3 figures have not been independently verified.

MarketDash has contacted Moderna by email for comment and will update this report if the company responds.

Earlier Phase 3 Success Raised Expectations

In August, Moderna and Merck announced positive topline results from INTerpath-001.

The combination demonstrated statistically significant improvements in recurrence-free survival and distant metastasis-free survival compared with Keytruda alone.

However, the companies did not disclose detailed efficacy figures at the time.

Phase 2b Data Set Higher Benchmark

At the June 2026 ASCO Annual Meeting, Moderna and Merck reported five-year follow-up results from their Phase 2b trial.

After a median follow-up of 60.3 months, intismeran plus Keytruda reduced the risk of recurrence or death by 49% compared with Keytruda alone.

The combination also lowered the risk of distant metastasis or death by 59%.

These findings reinforced earlier three-year follow-up results, which showed similar benefits.

However, differences in trial size, patient populations and follow-up periods limit direct comparisons. The complete Phase 3 results will provide more clarity on the vaccine's clinical benefits.

MRNA Price Action: Moderna shares were down 25.47% at $183.20 at the time of publication on Thursday, according to market data.