Researchers say they have successfully completed a trial of a personalised cancer vaccine that uses the same messenger-RNA technology as Covid jabs.
The experimental vaccine, made by Moderna and Merck, is designed to prime the immune system to seek and destroy cancerous cells.
Doctors hope work such as this could lead to revolutionary new ways to fight skin, bowel and other types of cancer.
Moderna and Merck called it "a new paradigm" moment.
Other pharmaceutical companies are looking to run similar studies.
But this is the first phase-IIb randomised clinical trial to test the investigational mRNA vaccine in patients.
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Patients taking Keytruda for advanced melanoma were less likely to die, or have the skin cancer reoccur, if they also had the jab, mRNA-4157/V940, Moderna and Merck said.
The findings, in 157 patients, have not yet been scrutinised by independent experts or regulators.
More trials will be needed to check how effective the treatment might be.
Moderna's chief medical officer Paul Burton said: "This is a significant finding. It's the first randomised-trial testing of an mRNA therapeutic in cancer patients.
"It's shown a 44% relative reduction in the risk of dying of cancer or having your cancer progress. That's an important finding and I think it has the potential to be a new paradigm in the treatment of cancer patients."
Tailormade to match each patient's cancer, the vaccine is very expensive to make - although, the company has not named a price.
Consultant colorectal surgeon and Cancer Research UK advanced clinician scientist at the University of Birmingham Mr Andrew Beggs said: "The use of the game-changing mRNA vaccine technology in increasing response to immunotherapy drugs is very exciting.
"The study used a personalised cancer vaccine to increase the effectiveness of immunotherapy in metastatic skin cancer, showing that it was well tolerated and seemed to reduce the rate of recurrence of the cancer.
"Although early data, it is very encouraging that this is a likely effective treatment option in the future.
"This advance is likely to have important implications for metastatic cancer patients in the future and opens a new therapeutic avenue for these patients."