For the first time, a novel treatment that trains the immune system to fight cancer has shown modest benefit in late-stage testing against the deadly skin cancer melanoma.

The approach is called a cancer vaccine, even though it treats disease rather than prevents it. In a study of about 180 patients already getting standard therapy, the vaccine doubled the number of patients whose tumors shrank, and extended the time until their cancer worsened by about six weeks.

“Although the differences may seem small, we need to understand that in many cancers, our progress has been built in small incremental changes over time,” said Dr. Len Lichtenfeld of the American Cancer Society.

Trends in the study suggest that the vaccine also may improve survival, but patients need to be followed longer to see if this proves true, said Dr. Douglas Schwartzentruber, cancer chief at Goshen Health System in central Indiana. He led the study and gave results Saturday at an American Society of Clinical Oncology meeting.

The National Cancer Institute developed the vaccine, which has not yet been commercially licensed. The institute sponsored the study, along with Novartis AG, which makes interleukin-2, the standard treatment.

The vaccine contains a substance found on the surface of many skin cancer cells. The idea is to help the immune system recognize this as a threat and provoke it to attack.