(Reuters) - A genetically engineered smallpox vaccine reduced the risk of death for patients with advanced liver cancer by nearly 60 percent in a mid-stage study, prompting the launch of a later-stage trial.

Scientists at institutions including the University of California, San Diego, and privately held biotech company Jennerex Inc presented Phase 2 trial data on Saturday showing that patients given high doses of the altered vaccine, known as JX-594, lived for a median of 13.8 months compared with 6.7 months for patients treated with one-tenth of that dose.

The small 30-patient study found that 66 percent of the high-dose patients were alive after one year, compared with 23 percent of the low-dose group.

Temporary flu-like symptoms were the main side effect seen in the trial, which was presented in San Francisco at a meeting of the American Association for the Study of Liver Diseases.

Scientists have been intrigued for decades with the idea of using viruses to alert the immune system to seek and destroy cancerous cells. That interest has taken off in recent years as advances in genetic engineering allow them to customize viruses that target tumors.

JX-594 is derived from a strain of the virus vaccinia, once commonly used to vaccinate children against smallpox.

"Viruses are inherently cancer selective and tumor cells are inherently susceptible to viral attack," said Dr. David Kirn, chief medical officer at Jennerex. "We enhance selectivity by further attenuating and weakening the virus in normal tissue."

He said the first patient has been enrolled in a Phase 2b study comparing JX-594 with standard care in 120 liver cancer patients who have stopped responding to Nexavar, also known as sorafenib, sold by Onyx Pharmaceuticals.

Patients in the trial will first be given an intravenous infusion of JX-594, followed by direct injections into the tumor. Dr. Kirn said the trial will also allow for more continuous dosing than in earlier studies.

Jennerex plans to launch next year a Phase 3 head-to-head trial comparing JX-594 with Nexavar and is conducting earlier-stage trials in other types of cancer.

Other forays into using engineered viruses include biotech giant Amgen Inc's deal in January to pay up to $1 billion for BioVex and its cancer drug development platform based on the herpes simplex virus.

Amgen said last month that it had completed enrollment in a Phase 3 trial of the therapy in melanoma patients.

Jennerex is primarily funded by investors from Canada and South Korea. European rights to JX-594 have been licensed to Transgene SA. Other regional licenses are held by Lee's Pharmaceutical Ltd for China and Green Cross Corp for South Korea.

(Reporting by Deena Beasley; Editing by Steve Orlofsky)