CAMBRIDGE, Mass. — Moderna shares soared as much as 147 percent on Wednesday and Merck rose about 13 percent after the two drugmakers said a personalized mRNA cancer vaccine slowed the return and spread of melanoma in a 1,137-patient Phase 3 trial, the first late-stage confirmation of an approach oncologists have chased for more than a decade.

The result, if the underlying data hold, would clear a path to Food and Drug Administration review for intismeran, a shot built from genetic material in each patient's own tumor and given alongside Merck's blockbuster immunotherapy Keytruda. Analysts have projected the combination could generate more than $6 billion in annual sales in melanoma alone, and considerably more if the same approach works in lung, kidney and bladder cancers.

What the trial showed

The study enrolled 1,137 patients whose melanoma had been surgically removed and randomly assigned them to receive either intismeran plus Keytruda or Keytruda alone for about a year. Merck and Moderna said the combination extended both "recurrence-free survival," the time patients live without their melanoma returning, and "distant metastasis-free survival," the time before new lesions appear elsewhere in the body. The companies described the improvement as "clinically meaningful" and said no new safety concerns emerged.

The underlying data have not been released. Merck and Moderna said full results will be presented at a future medical meeting and shared with regulators.

The Phase 3 study builds on a 157-patient Phase 2 trial whose five-year data, presented in June at the American Society of Clinical Oncology, showed a 49 percent reduction in the risk of recurrence and a 49 percent reduction in distant metastasis versus Keytruda alone.

How it is built

Intismeran is manufactured from a piece of each patient's tumor. Sequencing identifies the mutations that make the cancer distinct, and an algorithm selects which of the resulting neoantigens — abnormal proteins on the surface of cancer cells — the mRNA shot should train the immune system to attack. In the trial, the "needle to needle" turnaround was six weeks, according to Jane Healy, Merck's head of early oncology development.

"The beauty of mRNA technology is the scalability and the adaptability," Healy said. "It lends itself really well to this setting."

Moderna's president, Stephen Hoge, said the collaboration dates to the middle of the last decade. "I have a picture on my whiteboard from June of 2015 when we drew up how we were going to do it," Hoge said.

What it means for Moderna

Moderna stock has lost more than 80 percent of its value since the peak of the pandemic, and the Cambridge, Mass., company's outlook has grown steadily more dependent on its cancer program. Chief Executive Stéphane Bancel called Wednesday's readout "a pivotal moment for the field of cancer research" in the release announcing the results.

The political backdrop has been unforgiving. Health and Human Services Secretary Robert F. Kennedy Jr. cut $500 million in federal contracts for mRNA vaccine research and development last year, and Moderna has stopped calling its cancer product a vaccine, referring to it instead as an "individualized neoantigen therapy." Trump administration officials have signaled they remain open to mRNA in oncology, and the FDA this month approved Moderna's mRNA flu shot after initially rejecting the application under former vaccine chief Vinay Prasad.

The caveat

The Phase 3 trial was stopped at the first of several planned interim analyses, meaning the companies have not yet shown that the vaccine extends overall survival — the endpoint regulators and patients most want to see, and one that could take years to establish. The detailed numbers behind Wednesday's announcement have not been released or peer-reviewed, and Wednesday's coverage drew from the same Merck-Moderna press release; right-leaning outlets had not independently reported on the trial by press time.

Merck and Moderna said the study will continue to track overall survival. Investors will not see the underlying data until the still-unnamed medical meeting at which the companies plan to present them.