Drugmakers Merck and Moderna announced on 19 August 2026 that a personalised cancer vaccine, given alongside the immunotherapy drug Keytruda, significantly delayed the return of high-risk melanoma in a large phase three trial. The companies said patients who received the vaccine plus Keytruda after surgery stayed cancer-free for longer than those given Keytruda alone, and were also less likely to see the disease spread to other organs. Full data have not yet been published or independently peer-reviewed, and the treatment would still need regulatory approval before reaching patients on the NHS.
The trial, known as the interim analysis of the phase three programme, enrolled patients with high-risk stage two, three or four melanoma, ranging from large localised tumours to cancers that had already spread elsewhere in the body. The vaccine, called Intismeran, uses mRNA technology similar to that developed for some Covid-19 vaccines. Scientists take a small sample of a patient's tumour, sequence its DNA to identify specific mutations, and design a jab tailored to that individual's cancer, training the immune system to recognise and destroy any remaining tumour cells.
How many patients took part in the trial?
The original description of the study put enrolment at more than 1,000 patients with high-risk melanoma. However, according to CNBC, the late-stage trial actually involved more than 1,100 patients with higher-risk or advanced melanoma whose tumours had already been surgically removed. Both figures are reported here as neither source's account has been formally reconciled.
What exactly did the trial results show?
According to Merck's own release, a pre-specified interim analysis found the combination reduced the risk of recurrence or death by 49%, with a hazard ratio of 0.510 and a one-sided nominal p-value of 0.0075. Merck also confirmed the trial met both its primary endpoint of recurrence-free survival and a key secondary endpoint measuring how long patients remained free of distant metastasis, according to a company statement. Separately, a financial news agency's report described the results as showing the combination reduced both recurrence and spread of the disease in a large trial released on 19 August 2026.
Merck's release further specified that the trial population consisted of patients with completely resected stage IIB, IIC, III or IV cutaneous melanoma who had not previously received systemic therapy for the disease.
Why are researchers calling this a breakthrough?
Merck and Moderna describe Intismeran as a personalised vaccine because each dose is manufactured to match an individual patient's tumour rather than being a one-size-fits-all product. The companies are already testing similar personalised vaccines for lung, bladder and kidney cancers, suggesting the approach could extend well beyond melanoma if these results hold up under further scrutiny.
Stephane Bancel, chief executive of Moderna, said the findings mark an important moment for the field:
"Pivotal moment" in the field of cancer research.
Shares in both Moderna and Merck rose following the announcement.
How have cancer specialists responded?
Independent experts have welcomed the news while urging caution given that the data are still preliminary. Dr Talisia Quallo, head of prevention and early detection at Cancer Research UK, said:
"Shows promise" but said that the interim data still needs to be fully reviewed. "It's vital that research like this continues to be funded so that more people with cancer can have personalised treatment options to help them live longer, better lives."
Dr Lennard Lee, a consultant medical oncologist and associate professor at the University of Oxford, offered a similar assessment:
"This is certainly good news for patients with melanoma, but the study has much wider implications. It provides proof of principle that personalised cancer vaccines work."
What is the background to this research programme?
The combination therapy has been in development for several years. Merck and Moderna have previously said the vaccine, formerly known by the code mRNA-4157/V940, received Breakthrough Therapy Designation from the US Food and Drug Administration and PRIME status from the European Medicines Agency when used with Keytruda in high-risk melanoma patients following complete surgical removal of their tumours, according to an earlier Merck announcement. The phase three study itself was designed as a global, randomised, double-blind comparison of the vaccine plus Keytruda against Keytruda alone in patients with resected high-risk stage IIB-IV melanoma, according to a separate trial initiation announcement.
What happens next?
Merck and Moderna have indicated that regulatory filings are the next step following this interim readout, with the companies expected to follow the standard approval pathway before the combination could become available to patients, according to the financial news agency's report. The companies' prior statements also indicate that follow-up analyses will continue as the final clinical data package is prepared for submission to regulators, based on the earlier Merck release. Even once regulators in the United States and Europe review the submission, the treatment would still need to go through a separate assessment process before it could be offered on the NHS.
Key Facts
- Merck and Moderna announced interim results on 19 August 2026 for the vaccine Intismeran combined with Keytruda in high-risk melanoma.
- The trial met its primary recurrence-free survival endpoint and a key secondary distant metastasis-free survival endpoint.
- Merck reported a 49% reduction in the risk of recurrence or death, with a hazard ratio of 0.510.
- Trial enrolment is described as more than 1,000 patients in the original announcement and more than 1,100 patients according to a CNBC report.
- The vaccine previously received Breakthrough Therapy Designation from the FDA and PRIME status from the European Medicines Agency.
