One injection of experimental gene therapy stopped spontaneous bleeding in nine hemophilia patients for anywhere from two months up to a year, a Philadelphia-based research team announced Saturday.

Though the study was small, with a fairly short follow-up period, researchers were cautiously optimistic that the treatment could be effective for years, meaning that patients with this type of the disease — hemophilia B — would not only be free from painful internal bleeds in their joints, but also would no longer need to give themselves weekly infusions of costly clotting factors.

The study was sponsored by Spark Therapeutics, which was spun off from Children's Hospital of Philadelphia in 2013, and Pfizer Inc.

The treatment consists of one injection of nonreplicating viruses that are engineered to carry the genetic recipe for clotting factor nine, which is deficient in people with hemophilia B.  The results were presented at the annual meeting of the American Society of Hematology in San Diego.

In addition to preventing bleeds, the treatment did not trigger problematic responses in the immune system, an issue that had arisen in earlier research. Two of the nine patients experienced a mild immune response, but it was safely managed with steroids, the researchers said.

The apparent reason for the lack of immune response was that the treatment contained a lower dose of the genetically engineered viruses than past versions, but they carried the recipe for a more potent form of the clotting factor protein, said lead study author Lindsey A. George, a hematologist at CHOP.

"The idea here is, you can have less protein but have more activity," she said.

The treatment was based on years of basic scientific research by senior author Katherine High, who was at CHOP at the time and now is president and chief scientific officer of Spark.

All nine patients were men with levels of clotting factor so low, they were at risk of spontaneous bleeds. After the treatment, all had levels high enough to avoid that complication and to allow normal physical activity.

One of the patients thought he might have experienced an internal bleed two days after receiving the treatment and gave himself an infusion of clotting factor, but the bleed was not confirmed. Otherwise, the nine have experienced no spontaneous bleeds for a collective period lasting more than 4 1/2 years, the researchers said.