Treating a patient with concentrate costs $300,000 a year, with a possible lifetime cost of $20 million, but the single required injection of the new delivery virus costs just $30,000, Dr. Katherine P. Ponder of the Washington University School of Medicine in St. Louis notes in her commentary in The New England Journal of Medicine, calling the trial “a landmark study.”

The patients have continued to produce their own Factor IX for up to 22 months, said Dr. Edward G. D. Tuddenham, director of the Hemophilia Center at the Royal Free Hospital in London. One patient, a geologist, had a good response at first, but his level of Factor IX has declined to 1 percent of normal, the level at which the disease kicks in.

“We attribute this to the fact that he had an inflammation, and although we treated it promptly, we should have been quicker off the mark,” Dr. Tuddenham said.

The patient cannot be injected again with the same virus because his immune system is now primed to attack it. “He’s very philosophic about it, but he’s a scientist, and his motivation is to help the science,” Dr. Tuddenham said.

Twenty more patients will be treated to assess the best dose of the virus, the goal being the highest dose that does not set off an immune system attack, Dr. Tuddenham said. “We are pretty close to the sweet spot,” he said. If all goes well, a genetic treatment for hemophilia B “could be available for widespread use in a couple of years.”

In a trial in 2006, a patient injected with a corrective gene produced his own Factor IX but only for 10 weeks. The designer of that treatment, Dr. Katherine A. High of Children’s Hospital of Philadelphia, said the new therapy had worked because the delivery virus had been made more efficient and because the research team had treated the patients with steroids to suppress immune system attacks on the virus.