A customized vaccine from Merck and Moderna kept patients cancer-free for longer than standard treatments, after their melanoma skin cancers had been cut out. It is the first Phase 3 trial success for a cancer treatment vaccine, meaning the companies can now ask regulators to approve the treatment for marketing.

The companies said Wednesday morning that the combination of their intismeran autogene vaccine and Merck's immunotherapy drug Keytruda outperformed treatment with Keytruda alone, in preventing melanoma recurrence in the 1,100 patient study.

Moderna shares surged 65% in Wednesday's premarket session, while Merck advanced 7.1%.

While reporting the top line news that the vaccine made a "clinically meaningful" and statistically significant difference, the companies are saving the data for a coming medical meeting. They will also confer with U.S. regulators about filing for approval of the combination treatment.

A successful cancer vaccine would be an important new market for Moderna, whose investors have been impatient for it to expand beyond its Covid prevention vaccine. Moderna stock rose steeply in June, when the two companies reported five year follow-up data on their Phase 2 trial in melanoma. Cancer recurrence or death was halved, among patients who had received the vaccine-Keytruda combination.

A vaccine will also be important for Merck, which is looking for products to replace the revenue it will lose in the next few years, when patents roll off for Keytruda-the world's best selling drug. Merck sold $32 billion worth of Keytruda last year.

Researchers have searched for decades to find vaccines that might help the immune system better fight cancer. The results were disappointing, until now.

Earlier this month, scientists at Merck and Moderna told Barron's that their vaccine treatment is different because patients receive it earlier in their disease, as an adjuvant to surgery and when the cancer is more vulnerable to immune system defenses. Using Moderna's messenger-RNA technology, the companies individualize the vaccine to flag dozens of mutations that are unique to each patient's tumor.

That customization makes the product quite different from the off-the-shelf drugs and vaccines previously sold by the companies. But Moderna has automated the process and analysts believe it can scale up to serve billions of dollars worth of annual sales.

Besides melanoma, the companies are also testing their vaccine treatment against cancers of the lung, bladder, kidney, stomach and pancreas.

"Today's results represent a landmark moment for adjuvant melanoma treatment," said the Phase 3 study's leader, Georgina Long, an oncology researcher at the University of Sydney in Australia.