CHICAGO (Reuters) - US researchers are targeting a deadly brain cancer by taking aim at a common virus active in most people with these tumors.

The hope is to trick the immune system into attacking the cancer. So far, it seems to be helping, researchers at Duke University in North Carolina said on Monday.

Sixteen patients with the brain cancer glioblastoma multiforme treated with a vaccine that attacks the virus have seen significant delays in tumor progression — the time from when their tumor is cut out by a surgeon to when it starts to grow back.

In people treated with the vaccine, that time to progression has doubled, jumping from the typical six months to more than 12 months. Overall survival has increased from about 14 months to more than 20 months.

"We have seen patients who are beyond 20 months with no signs of their tumor growing," Dr. Duane Mitchell, a cancer researcher at Duke, said in a telephone interview.

Mitchell said prior studies found that human cytomegalovirus, a close cousin of the herpes virus, was active in about 80 percent of patients who are newly diagnosed with this type of cancer.

And, they found it was active in their brain tumors, but not in the rest of the brain.

"We are using the presence of the virus within these tumors as a target for a new immunotherapy," said Mitchell, who led the study.

The researchers make the vaccine from cells in the patients' blood. The idea is to activate their immune systems, which will then attack and kill the tumor cells.

Patients in the study got the vaccine in conjunction with the chemotherapy drug temozolomide, which seems to further stimulate the immune response to the vaccine.

Mitchell said some 70 to 90 percent of Americans are exposed to the human cytomegalovirus in childhood. It typically remains dormant in people with healthy immune systems.

He said it is not clear whether the virus plays a role in promoting the cancer, but its presence in the tumors make the virus a useful target for immune-based therapy.

While the study needs to be tested at different centers and in a much larger group of patients, Mitchell said it does offer a bit of hope for patients with this cancer, which has no effective treatments.

"Seeing a doubling in the time to progression is very encouraging to us that in large-scale trials this will actually have a benefit," Mitchell said.