According to data released by drugmakers Moderna and Merck on Thursday, the vaccine cut the risk of dying from melanoma or having it return by 42 per cent when combined with Merck's Keytruda immunotherapy, which is among the world's biggest-selling medicines.
The vaccine was custom-brewed for each patient to match their tumour.
The data from the Phase 3 trial of more than 1100 patients also showed the combination cut the risk of the cancer's spread by 38 per cent.
It is the first data to show that the combination of a vaccine, based on mutations unique to each patient's tumour, plus a drug that helps the immune system fight cancer can keep the disease at bay.
Moderna shares closed more than 17 per cent lower after the results leaked on an oncology website earlier on Thursday, but have been surging all year.
Moderna stock has particularly soared since the companies announced the trial's success in August without the actual data, nearly quadrupling through Wednesday's close.
Merck shares closed down about 1.8 per cent.
The drug combination is already being tested in multiple other cancers, including kidney, bladder and lung cancer.
But some analysts said the data may raise concerns among investors about how broadly the technology may extend to other cancers.
"We see optimism waning on pricing in multi-billion-dollar franchises across oncology," RBC analyst Trung Huynh said.
"This data makes us less optimistic as melanoma was supposed to be the best case."
Keytruda works by removing a natural brake cancer cells use to hide from immune system attack, while the vaccine instructs the patient's cells to produce copies of specific mutations for the immune system to seek and destroy.
Moderna chief executive Stephane Bancel said the data signalled a new chapter in cancer treatment.
"With this ability to create new T cells that are specific to your tumour ... the clinical impact is very profound and as we saw in the Phase 2 (trial), very lasting," Bancel said.
The companies are preparing to file for US approval and expect the vaccine, known as intismeran autogene, to launch in 2027, Bancel said.
Bancel said he believed the vaccine should add to efficacy "everywhere that Keytruda works", but it remained to be seen how much it would improve results.
Keytruda is approved for 19 types of cancer.
The executive also said intismeran could work on its own as a treatment in early-stage cancers.
The Phase 3 results, based on patient data for the first two years after treatment, are scheduled to be presented on Saturday at the European Society for Medical Oncology meeting in Madrid.
Among those who received the vaccine in addition to Keytruda, 82.2 per cent were still cancer-free after two years, compared with 70 per cent of patients who received Keytruda alone.
Dr Hussein Tawbi, a melanoma expert and trial investigator at the University of Texas MD Anderson Cancer Centre, said this was highly statistically significant.
"It is not by chance," he said.
With AP
