mRNA-4157 and KEYNOTE-942: What the First Positive Neoantigen Vaccine Trial Actually Showed

By Lociven · NeoantigenLab · July 2026

Figure 1. KEYNOTE-942 recurrence-free survival: mRNA-4157 + pembrolizumab vs pembrolizumab alone (HR 0.56).

In December 2023, Moderna and Merck reported that mRNA-4157 combined with pembrolizumab reduced the risk of recurrence or death by 49% compared to pembrolizumab alone in resected high-risk melanoma. It was the first randomized trial to show that a personalized neoantigen vaccine significantly improves outcomes in solid tumors.

The result shifted the field. Before KEYNOTE-942, neoantigen vaccines were an interesting idea with limited clinical evidence. After it, every major pharma company accelerated their personalized vaccine programs. This post breaks down what mRNA-4157 is, how the trial was designed, what the data actually show, and what it means for the field going forward.

What is mRNA-4157?

mRNA-4157 (now called V940) is a personalized mRNA cancer vaccine. For each patient, the process works as follows:

  1. Tumor sequencing: WES of resected tumor and matched normal identifies somatic mutations
  2. Neoantigen prediction: Computational pipeline selects up to 34 neoantigens based on predicted HLA binding, expression, and clonality
  3. mRNA synthesis: A single circular mRNA construct encoding all selected neoantigens is manufactured — the same lipid nanoparticle technology used in Moderna's COVID-19 vaccine
  4. Administration: Nine doses over approximately one year, combined with pembrolizumab

The manufacturing turnaround from biopsy to first dose was approximately 4–8 weeks in the trial. This is operationally remarkable — it means the vaccine can be deployed in the adjuvant setting before recurrence.

KEYNOTE-942: Trial design

The phase 2b trial enrolled 157 patients with resected stage IIIB–IV melanoma (high recurrence risk). Patients were randomized 2:1 to:

  • Arm A (107 patients): mRNA-4157 + pembrolizumab
  • Arm B (50 patients): pembrolizumab alone

Primary endpoint: recurrence-free survival (RFS). Secondary endpoints included distant metastasis-free survival (DMFS) and overall survival.

At the 3-year follow-up (presented at ASCO 2024):

  • RFS: 74.8% vs 55.6% at 3 years (HR 0.51, 95% CI 0.29–0.91)
  • DMFS: HR 0.38 (95% CI 0.17–0.86, favoring combination)
  • No new safety signals; the combination was well tolerated

Why melanoma first?

Melanoma is not an accident. It is one of the highest-TMB solid tumors — a median of 10–15 mut/Mb — which means more neoantigens per patient. It also has a well-established response rate to checkpoint blockade, making it easier to show additive benefit. And the adjuvant setting in resected high-risk melanoma has a well-defined endpoint (recurrence) with relatively short follow-up requirements.

The question the field is now asking is: does this work in lower-TMB tumors? Ongoing trials are testing mRNA-4157 in lung cancer, colorectal cancer, and bladder cancer — tumor types with more variable TMB and less established checkpoint response.

Mechanism: why combine with pembrolizumab?

The combination is not arbitrary. The mechanistic logic:

The vaccine primes neoantigen-specific T cells. mRNA-4157 encodes up to 34 neoantigens. Each neoantigen peptide is processed and presented by dendritic cells after LNP delivery, activating CD8+ and CD4+ T cells that are specific for tumor neoantigens.

Pembrolizumab removes the brake. Tumor-infiltrating T cells often upregulate PD-1 as a consequence of chronic antigen exposure. PD-L1 on tumor cells engages PD-1 to suppress T cell function. Anti-PD-1 blocks this interaction, allowing the vaccine-primed T cells to remain functional in the tumor microenvironment.

Without the vaccine, pembrolizumab can only reinvigorate T cells that pre-exist in the tumor. With the vaccine, it can reinvigorate a larger, newly expanded pool of neoantigen-specific T cells.

What we still don't know

Which of the 34 neoantigens actually matter? The trial does not tell us. Each patient's vaccine is different, and the contribution of individual neoantigens to the immune response is not reported. It is possible that 2–3 neoantigens drive the response and the rest are passengers.

Is this checkpoint + vaccine, or just better checkpoint? The 49% RFS improvement is impressive, but KEYNOTE-942 is a phase 2b trial with 157 patients. The phase 3 trial (KEYNOTE-942 expanded into V940-001) will be more definitive. If the effect size shrinks in the larger trial, interpretation becomes complicated.

Does manufacturing speed matter? A 4–8 week turnaround is fast for a personalized biologic, but it means patients start pembrolizumab alone for the first cycles. Whether earlier vaccine delivery would improve outcomes is unknown.

Cost and access. Personalized mRNA vaccines require individual manufacturing runs. The cost per patient is substantial. At scale, the economics depend heavily on manufacturing efficiencies that don't yet exist.

What this means for your research

If you work in tumor immunology, the KEYNOTE-942 result validates the core neoantigen vaccine hypothesis — that patient-specific neoantigens can drive clinically meaningful immune responses. That matters for:

  • Studies asking which neoantigen features predict immunogenicity
  • TIL expansion protocols — the T cells you are expanding are more likely neoantigen-reactive than previously appreciated
  • Biomarker discovery — what predicts who responds to the combination?
  • Combination strategies with other checkpoint targets (CTLA-4, LAG-3, TIGIT)

The next post covers pVACseq — the open-source pipeline that automates neoantigen prediction from WES data, and how to interpret its output without a bioinformatics background.

Tags: mRNA-4157, neoantigen vaccine, KEYNOTE-942, pembrolizumab, personalized cancer vaccine, Moderna, melanoma, checkpoint inhibitor

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Neoantigen selection criteria, 25-mer peptide design, codon optimization, and concatenation order — the design framework behind mRNA-4157 itself, fact-checked against current tool docs and trial data.

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