The deadliest and most common type of brain cancer has a strange bedfellow: cytomegalovirus, a kind of herpes present in about 80 percent of the U.S. population. Now scientists are exploiting this coincidence to treat the cancer with a vaccine that targets the virus and slows tumor regrowth.

In 2002 scientists showed that cytomegalovirus, or CMV, was active in the brain tumors but not the surrounding healthy tissue of all 27 patients they tested who had glioblastoma multiforme. CMV is dormant and undetectable in most people. Neuroscientist Duane Mitchell of Duke University Medical Center and his colleagues confirmed in 2007 that CMV is active in at least 90 percent of glioblastoma tumors. Now Mitchell’s team has developed an experimental vaccine that triggers the immune system to attack CMV, thereby attacking its tumor tissue home. As reported at the American Society of Clinical Oncology meeting in June, the vaccine, together with radiation and chemotherapy, prevented the brain tumor from reemerging after surgery for 12 months as compared with the typical six to seven months with no vaccine. Patients’ average life span increased from 14 months to more than 20.

So does this herpes virus cause cancer? The answer is unclear: tumor cells may simply be a fertile ground for growing the virus, as cells such as these often lack the normal immune functions that suppress CMV reproduction. But University of Wisconsin–Madison researchers reported in May that the virus has the ability to take over a cell’s braking mechanism and cause uncontrolled reproduction. Even so, the numbers do not seem to add up: four of five Americans has CMV, but only about one in 30,000 ends up with glioblastoma. And a small number of glioblastoma patients do not have CMV in their tumors.

“Most evidence to date does not support CMV being a cancer-causing virus,” Mitchell says. Don Diamond, a virologist at the City of Hope Cancer Center near Los Angeles, agrees: his extensive research on CMV and cancer has convinced him the virus does not cause tumors. But for patients it does not matter whether the connection between herpes and brain cancer is causal or not—the vaccine appears to work. Mitchell hopes to have the vaccine ready for market in a few years.