Doctors explain how Moderna

Moderna is touting a "landmark moment" in its experimental melanoma treatment, saying its vaccine could help prevent the deadly skin cancer from returning or spreading.

The groundbreaking clinical trial studied over 1,100 people whose melanoma was in advanced stages, taking a piece of a patient's tumor and using it to build a shot, CBS News medical correspondent Dr. Céline Gounder explained.

The results came from a Phase 3 trial, the last stage before a Food and Drug Administration review.

Gounder, who has had melanoma herself but noticed it very early, said the trial is "a big win for mRNA vaccines against cancer." Following the news, Moderna's stock more than doubled on Wednesday morning.

After a piece of the patient's tumor is removed, it is analyzed to identify the unique mutations present, CBS News chief medical correspondent Dr. Jon LaPook said. Artificial intelligence is then used to help scientists figure out which of the hundreds of mutations should be included in the vaccine. 

Like other mRNA vaccines, Moderna's melanoma vaccine delivers synthetic mRNA into cells, delivering a "one-two punch" to melanoma, LaPook said.

Moderna's mRNA vaccine — used in combination with Merck's cancer immunotherapy drug Keytruda — is built from a patient's own tumor, which has its own genetic sequence, so each dose is personalized.  

"Cancer can put a cloak of invisibility around itself so the immune system can't see it, and the first punch is a drug called Keytruda, which removes the cloak," LaPook explained.

Meanwhile, the patient's cells read the mRNA and build a protein, breaking the messenger down. That protein then trains the immune system to learn to attack cancer cells.

"Think of it like giving a scent to a bloodhound. The vaccine gives the immune system the scent of melanoma so the immune system can hunt it down and kill it," LaPook said.

For patients whose melanoma had gone deeper, or reached the lymph nodes, Moderna and its drug development partner Merck said the novel treatment showed "statistically significant and clinically meaningful improvement" in helping patients remain cancer-free for longer.

For those participating in the trial, surgeons had already removed their tumors, and everyone had been treated with Keytruda, the standard immunotherapy. But some patients were also given the mRNA melanoma vaccine. That group stayed cancer-free longer, the trial found.

Melanoma is the fifth most commonly diagnosed cancer in the United States, with about 112,000 new cases each year. The American Cancer Society projects that about 8,500 people in the U.S. will die from melanoma this year.

Jon LaPook and Dr. Céline Gounder contributed to this report.