An experimental personalized mRNA vaccine combined with Keytruda reduced the risk of melanoma returning after surgery or causing death by 42% compared with Keytruda alone, according to results from a late stage clinical trial. The findings could open a new path for treating the disease.

“This new vaccine is something that we’ve never really seen before in terms of the efficacy that it has demonstrated and the way that it works,” Dr. Michael Postow, chief of the melanoma service at Memorial Sloan Kettering Cancer Center, told CNN.

The vaccine is currently called intismeran autogene and has not yet received a brand name. Merck and Moderna, its developers, released the findings early on Thursday after reports that the data had appeared online and then been removed.

The results are scheduled for presentation Saturday in Madrid at a meeting of the European Society for Medical Oncology. The companies also said they are engaging with regulatory authorities.

Merck and Moderna are testing the technology in other forms of cancer, including lung and pancreatic cancers. The experimental treatment uses the same mRNA technology behind Moderna’s Covid-19 vaccine, but directs the immune system against cancer cells rather than a virus.

Each vaccine is personalized according to the genetic signature of an individual patient’s tumor. After the cancer is removed through surgery, a tumor sample is sequenced, allowing the vaccine to be designed around markers most likely to provoke an immune response.

“This personalized cancer vaccine is a specific type of treatment that boosts the body’s immune response only against the cancer cells that are there,” Postow said, “and therefore it is a much more targeted immune boost directly against that patient’s individual tumor.”

Postow worked with patients in the trial, including Beth Ryzyk, who was diagnosed with stage IIIC melanoma in December 2023. Her cancer had reached a certain thickness in the skin and spread to her lymph nodes, increasing the danger that it could return after surgery.

Patients in that situation generally receive treatment after surgery to reduce the risk of recurrence, an approach known as the adjuvant setting. Keytruda is commonly used and works by enabling the immune system to attack cancer cells that may previously have remained hidden.

Keytruda received approval in 2014 and was used to treat President Jimmy Carter after melanoma spread to his liver and brain. Carter announced in December 2015 that he was cancer free.

Ryzyk was among 1,136 participants, all of whom received Keytruda. She still does not know whether she also received the experimental vaccine or a placebo, although she suspects she received the vaccine because she felt “a little sick, just a little flu and kind of down the next day.”

The vaccine or placebo was delivered through an intramuscular injection every three weeks for as many as nine doses. Keytruda was administered through an intravenous infusion every six weeks for up to about a year, meaning the treatments were not always given together.

More than a year after completing treatment in the trial, Ryzyk remains cancer free. “I’m good; sometimes I even forget about it,” she told CNN. “It’s always in the back of your mind … but they’re watching.”

The 42% reduction in the risk of recurrence or death was measured after a median of about two years following treatment. The trial will continue while researchers evaluate how long the effect lasts and whether the treatment improves overall survival.

The companies said it is still too early to determine whether patients receiving the combination live longer, although they reported a favorable trend. Despite the encouraging trial findings, investors reacted sharply when the data were released Thursday.

Moderna’s stock fell 18% before the market closed, while Merck’s shares declined 1.8%. Trung Huynh of RBC Capital Markets said the results were not quite as strong as findings from an earlier, smaller trial, but predicted that the FDA would probably approve the therapy next year.

Investors are also watching for proof that the approach works against cancers beyond melanoma. Moderna’s stock more than doubled in August after news that the trial had succeeded, and even after Thursday’s decline, shares remained up more than 500% for the year, according to William Blair analyst Myles Minter.

Ryzyk said she hopes other patients can benefit from the treatment and that its eventual price will remain within reach. After completing therapy last spring, she returned to traveling with her husband and enjoying retirement, more than a year after finishing the trial without a cancer recurrence.