Each patient's melanoma leaves behind a different molecular fingerprint—and a new vaccine is designed to teach the immune system what that fingerprint looks like. In an international Phase III trial, adding the personalized vaccine intismeran to pembrolizumab after surgery lowered the risk of melanoma returning or causing death by 42%, compared with pembrolizumab alone.
The findings, presented by Professor Georgina Long at the European Society for Medical Oncology Congress 2026 in Sydney, could offer a new way to help people with high-risk melanoma after surgery. Even when doctors remove a tumor, some patients face a serious chance that the cancer will return or spread. The trial focused on people whose melanoma was at stages IIB to IV and who had not previously received cancer drug treatment.
A total of 1,137 patients took part. Researchers compared pembrolizumab alone with pembrolizumab plus intismeran, a therapeutic vaccine made for each patient from a sample of their own tumor. Unlike vaccines that aim to prevent an illness before it happens, this treatment is given after a cancer diagnosis to help lower the chance of the cancer coming back.
The vaccine uses mRNA technology to help the body recognize abnormal proteins, called neoantigens, that make each tumor distinct. Long described the process as creating a “wanted poster” of the cancer: the patient's cells display the tumor’s unique markers so immune cells can learn what to attack. Pembrolizumab works alongside the vaccine by blocking signals that can stop immune cells from attacking cancer.
In the trial, the combination reduced the risk of recurrence or death by 42% compared with pembrolizumab alone. Among patients who remained free of recurrence for more than six months after treatment began, the reduction was 58%. These percentages describe differences in risk between the trial groups; they do not mean that every patient's personal chance of recurrence fell by the same amount.
The vaccine did not increase pembrolizumab’s side effects, and treatment was generally well tolerated. Some patients had short-lived, mild effects from the vaccine, including pain where it was injected, headaches and chills. Long called the results a “foundational shift” in cancer treatment, saying the trial showed that care based on the unique features of a patient's tumor can work with immunotherapy to target cancer cells left behind after surgery.
The results mark an important step, rather than a promise that melanoma will not return. The trial tested a specific treatment combination in people at high risk after surgery. Its findings suggest that tailoring treatment to a tumor’s own biology may help more patients stay free of recurrence, while offering researchers a model to explore in cancer care. For people facing melanoma, the central finding is clear: a vaccine built from their own tumor, used alongside an existing immunotherapy, reduced the risk of recurrence or death in a large international trial.