An experimental messenger RNA cancer vaccine developed by Moderna and Merck & Co paired with Merck's Keytruda cut the chance of recurrence or death from melanoma by half after three years, showing that benefits demonstrated a year ago have held up over time.

The combination of the personalized cancer vaccine and Merck's blockbuster immunotherapy cut the risk of recurrence or death of the most deadly skin cancer by 49% compared with Keytruda alone in the midstage trial, the companies said.

The results come at a median point of three years into the study involving 157 patients with stage III/IV melanoma whose tumors were surgically removed before being treated with either the drug/vaccine combination or Keytruda alone with the aim of delaying disease recurrence.

A year earlier, the study had shown a 44% reduction of recurrence or death.

"The durability of the responses is really strong, they're essentially rock solid through this time," Moderna President Stephen Hoge said in an interview. "This is a pretty significant improvement, a pretty dramatic improvement over standard of care with just Keytruda alone."

The combination treatment has won U.S. breakthrough therapy and European Medicines Agency PRIME scheme designations, regulatory programs that aim to speed development of innovative treatments. Still, Hoge said that even with the new data it would be some time before the companies can file for approval of the treatment.

The vaccine is custom-built based on an analysis of a patient's tumors after surgical removal. The vaccines are designed to train the immune system to recognize and attack specific mutations in cancer cells.

Moderna is currently building a dedicated facility in Massachusetts to produce the vaccine at commercial scale, which it hopes to finish sometime next year.

"We need to make sure that we have that near completion before we could even contemplate asking for approval," Hoge said.

He said the companies are engaging with regulators globally on the pathway for approval for the product.

They have already begun a comfirmatory late-stage trial for the combination in melanoma, as well as one in non-small cell lung cancer that is already enrolling patients.