The numbers first: 1,137 patients, split two to one, all with melanoma that surgeons had already removed. On Wednesday, 19 August 2026, Merck and Moderna said a personalized mRNA vaccine, added to Merck’s Keytruda, held back the cancer’s return and its spread to other parts of the body better than Keytruda did on its own.
The bottom line: this is the first randomized Phase 3 trial built to prove that a neoantigen vaccine works, and by the companies’ account it did. The vaccine is called intismeran autogene, also known as mRNA-4157 or V940. The full data has not been released.
How the trial ran
Patients had cutaneous melanoma at stage IIB, IIC, III or IV, and all had a complete surgical removal of their tumors. Two out of every three got intismeran plus Keytruda. The rest got Keytruda alone. Intismeran went in every three weeks for up to nine doses, Keytruda every six weeks, and treatment ran about a year.
Interim results showed significant and clinically meaningful gains on both of the trial’s main goals, the companies said. They also reported no new safety concerns, with the profile matching earlier studies. Patients stay in follow-up, and the tracking includes how long they live.
Merck and Moderna plan to present the results at an upcoming medical meeting and to discuss possible approval with regulators.
One vaccine per tumor
This is not a one-size-fits-all shot. Each dose is custom-built. Researchers take a sample of the patient’s tumor and study its mutations, then pick neoantigens, abnormal proteins tied to the cancer that the immune system can target. The vaccine carries synthetic mRNA encoding up to 34 of them, packed in lipid nanoparticles. Once it is given, the instructions let those targets show up to the immune system, with the aim of triggering T-cell responses against cancer cells that carry them. Keytruda blocks the PD-1 receptor, which helps switch on T cells against tumors.
Development took more than a decade. Scientists first tried mRNA vaccines in mice in the 1990s, long before the COVID-19 shots made the technology familiar.
What came before
An earlier Phase 2b study gave the first hint. After five years, the combination cut the risk of the cancer returning, or of death, by 49 percent compared with Keytruda alone. The risk of spread to other parts of the body, or death, fell by 59 percent.
Georgina Long, the study’s principal investigator, called the new result “a landmark moment for adjuvant melanoma treatment.” She said intismeran with pembrolizumab “has the potential to establish a new treatment paradigm in the adjuvant melanoma setting, helping patients remain cancer-free for longer.”
Moderna and Merck are now testing the approach in nine Phase 2 and Phase 3 trials, covering lung, bladder and kidney cancers. Patients with melanoma who have questions about their own care should talk to their doctor.
Sources
- Moderna and Merck say mRNA cancer vaccine succeeded in late-stage melanoma trial
- US' custom mRNA vaccine scores Phase 3 win against deadly skin cancer
